The game features three modes. "Postal 2" which has you playing Monday through Friday, "Apocalypse Weekend" which is the expansion that added Saturday and Sunday and "A week in paradise" combining both into one playthrough.
Achievements related to playthroughs:
Getting "40 Year Old Virgin" requires the player to beat the "Postal 2" game mode in under 1h and 30m. Getting to know the town inside and out helps as to cut down on time spent getting lost. Set a lower difficulty to help with this achievement.
Getting "Anustart!" requires the player to beat the "Postal 2" game mode with no kills. Every single errand can be completed without killing anybody. Whether it's not robbing the bank, confessing your sins or visiting Uncle Dave or just not shooting people, there is always a way to handle a situation peacefully.
Getting "CAN YOU DIG IT?!" requires the player to beat the "Postal 2" game mode getting over 30 kills with only a shovel. Playing on "Liebermode" helps since on that difficulty all of the NPCs only have melee weapons.
Getting "Planet of the Apes: The Musical" requires the player to complete the game on "Hestonworld" difficulty. One of the harder difficulties in the game.
Getting "Scientology Level: OT VIII" requires the player to complete "A week in paradise" game mode on "Postal" difficulty. Being a Postal god is not easy, but this achievement requires you to be one. Every single NPC hates you and is actively trying to kill you. Did I mention you can't store healing items? It's hard and it's brutal, a really slow crawl through the game, having to kill everyone you see. RWS was kind enough to give you an unlimited "Bass Sniffer" on this difficulty but it doesn't lower the difficulty all that much. Oh and one save per map...
Getting "I should buy a boat" requires the player to pick up the newspaper every day. On Monday through Friday the newspaper spawns near the player, so just have a quick glance around yourself and you should see it (in front of the RV). Saturday and Sunday require some walking to get to, but due to the linear nature of the expansion levels you can't miss it.
Achievements related to specific days:
Monday:
Getting "Lieberman is our Leader!" requires the player to kill all of the video game protesters in the RWS building. Picking up the paycheck triggers a cut scene where a group of protesters storm into the RWS building wielding weapons. Even though you got fired, at least help out a little bit.
Getting "Fur Sure!" requires the player to kill "Vince" and "Mike J". During or after the protesters enter the building the player is able to get this achievement.
Getting "There's always money in the banana stand" requires the player to use the hidden exit right outside the bank vault. Robbing the bank is quicker than waiting in line. Storm the vault, grab the cash and when leaving turn left on the vault exit and you will see a bookshelf. Kicking the bookshelf moves it, revealing a secret passage out of the bank (there are a few goodies on the way out).
Getting "Someone's got a case of the Mondays!" requires the player to complete the first day. This achievement is gotten just from playing.
Tuesday:
Getting "And THAT'S why you never get out of the tree" requires the player to kill all of the protesters that set the library on fire. After returning the book a cut scene plays showing the protesters lighting the library on fire. You will encounter all of them while escaping the burning library.
Getting "It's OK, we got Greenlit anyway" requires the player to kill a person after they refuse to sign the petition. When asking a person to sign your petition, there is a chance of them declining. If a person refuses you can then ask them two more times. Asking them the third time always makes them run away screaming due to a threat given by The Dude.
Getting "Officer McLovin'" requires the player to ask an officer to sign the petition. Police officers are common throughout the game. Found on all of the streets of Paradise and in the police station. Walk up to one with a clipboard and press the attack key to ask them to sign your petition.
Getting "One time, at band camp..." requires the player to save the marching band from getting blown up by a terrorist. When Tuesday starts there will be a marching band parading down the street. The band continues up the street and is eventually blown up by a suicide bomber and to get the achievement you must stop him.
Getting "I'm not even supposed to be here today!" requires the player to find the June 2016 Easter egg. In the top floor of the mall there is a fenced-out part with a sign on it reading "Coming June 2016". In June of 2016, RWS made good on their promise, and added an Easter egg here. After crawling through the hole, there will be a table with a VR Headset on it. Interacting with it makes The Dude put it on after which he is placed inside a virtual "Steme" (yes, Steme) store. Wait in line and purchase a copy of "Postal Redux".
Getting "C U Next Tuesday" requires the player to complete the second day. This achievement is gotten just from playing.
Wednesday:
Getting "I swear, I am NOT Marcellus Wallace" requires you to kill 10 bystanders while wearing the gimp outfit. After peeing on your dad's grave, a hillbilly knocks you out. After awaking in the factory, you soon find out you were put in a gimp suit and this prompts a new errand of going to the dry cleaners. Along the way kill 10 people and you get this achievement.
Getting "OMG, NEW MODERN FAMILY TONIGHT!" requires the player to complete the third day. This achievement is gotten just from playing.
Thursday:
Getting "FAB-U-LOUS!" requires the player to wear all three outfits. The first outfit is The Dude's regular set of clothes. The second is the gimp suit The Dude is forced to wear on Wednesday. The third outfit is the policeman's uniform. Getting the uniform is straight forward: break into the police station and head to the top floor and locate the locker room. In one of the lockers there is a uniform, pick it up and put it on.
Getting "You are not alone" requires the player to see an NPC "go postal". Enter the "Lucky Ganesh" and head upstairs. Enter the big living room. Three Taliban's will be there arguing. Soon enough, they attack each other.
Getting "Where's Mr. McGibblets?"requires getting the "Krotchy doll" and killing Krotchy. Following the mission "Get Krotchy" brings you to a toy store in the mall where all of the Krotchy dolls are sold out. Head down to the warehouse and grab one for yourself. On the way back you will face off against the real Krotchy.
Getting "It must be Thursday, I could never get the hang of Thursdays" requires the player to complete the fourth day. This achievement is gotten just from playing.
Friday:
Getting "Gary vs. a Giant Penis" requires you to find the Gary vs. Krotchy Easter egg. After the apocalypse starts, head to the mall. Right outside of "Toyz 'n tha hood" there is a standing ladder. I suggest saving here. Start kicking the ladder down the mall (the ladder is facing the way it's supposed to) being careful not to tip the ladder over, it is impossible to get them to stand after falling. After some kicking, you will reach a wall the height of the ladder and after climbing up, head left. There is an arena and inside are Gary and Krotchy.
Getting "SCREW that game!" requires the player to find the POSTAL III Easter egg. While getting the alternator from the junkyard, make sure to stop by a small body of dirty water. In one of the bushes around the water is a copy of "POSTAL III". Peeing on the game box grants this achievement.
Getting "Workin' for the Weekend" requires the player to complete the fifth day. This achievement is gotten just from playing.
Saturday:
Getting "Srew Bill Lumbergh!" requires the player to complete the sixth day. This achievement is gotten just from playing.
Sunday:
Getting "I swear, I thought it was a home fill" requires the player to complete the seventh day. This achievement is gotten just from playing.
Getting "Thanks for the money" requires the player to beat all seven days. This achievement is gotten just from playing.
Achievements that can be done on any day:
Throwing a sledgehammer on a fleeing NPC grants "Anger Management With Roger Clemens". The sledgehammer is an "Apocalypse Weekend" weapon and as such can be found on Saturday and Sunday. You can force an NPC to flee by hurting them or peeing on them (may cause them to puke).
Hitting a decapitated head over 50 meters with a shovel grants "A-Rod Who?". NPCs can be decapitated by attacking their neck or head with a slashing weapon. After decapitating an NPC, find a hill and try kicking the head off it. The shovel can be found around the starting area.
Smashing over 30 heads with a sledgehammer grants "Can't Touch This". The sledgehammer is an "Apocalypse Weekend" weapon and as such can be found on Saturday and Sunday.
Having your dog kill 30 NPCs grants "Cesar Milan is Gay". Being attacked or attacking forces the recruited dog to help you. You can recruit dogs by dropping (default "X") dog treats.
Having a police officer eat a pissed-on doughnut grants "Chapelle's Show". Police officers are common throughout the game. Found on all of the streets of Paradise and in the police station. Dropping doughnuts (default "X") lures police officers, and peeing on one of these doughnuts makes them vomit. Your neighbors might have some doughnuts.
Killing someone with a kick grants "Chuck Norris'd!". Damaging an NPC slightly helps you kick them to death.
Chopping 30 people in half with a scythe grants "Darth Maul'd!". The scythe is an "Apocalypse Weekend" weapon and as such can be found on Saturday and Sunday.
Severing all of the limbs of an NPC without them dying grants "Door Mat". This can be done with any slashing weapon (capable of dismembering). It can take some tries to do, but is not hard. I recommend hedge clippers.
Using your taser on 20 NPCs while wearing the police uniform grants "Don't taze me, bro!". The police uniform can be found on the top floor locker room of the police station. The taser can be found in the house in-front of your trailer or can drop after killing a police officer.
Unzipping your pants and having 3 female NPCs to laugh at it grants "Do you even Enzyte, bro?". Unzip your pants (default "R"), and walk around town presenting yourself.
Using a Bass Sniffer grants "Duck Dynasty". On Tuesday there's one in the shack in front of your trailer.
Smoking over 10 pipes and 10 tins of catnip grants "Fear and Loathing". This achievement has to be done in one play session which means saving and loading is allowed, while leaving the game resets the counter. Take note of all the pipes and catnip you find, it helps a bunch in this achievement.
Kicking a decapitated head grants "Finkle IS Einhorn!". To decapitate an NPC just use a slashing weapon on their neck.
Finding the secret Taliban base grants "Found 'em faster than GWB". Behind The Dude's camper there is a secret sewer entrance. There is a circuit box behind closed bars. Shooting the box opens the way forward. Continue down the sewers, through the canyon (being careful not to fall) into the base. Upon entering you should get the achievement. A video by GameTHAT showing the whole achievement.
Hiding from the police after reaching max wanted level grants "Friend of Dorothy". Putting on another set of clothes removes any wanted level you might have had.
Burning 5 NPCs with the same shot of napalm grants "GOOD MORNING VIETNAM!". Completing the Thursday objective "Get Napalm" gives you napalm. Point at crowd, shoot.
Entering the sewers behind The Dude's trailer grants "Gotta pay the Troll Toll". This achievement is gotten while doing the "Found 'em faster than GWB".
Kicking 30 dogs grants "Hello, Newman". If you see a dog, kick it.
Recruiting your first dog grants "Here, Wilfred". Dropping dog treats (default "X") near a dog recruits him.
Killing over 1,000 people grants "Hutton Gibson Can't Deny This!".
Committing suicide grants "I Am Legend". The default key to suicide is "K".
Killing a Taliban by suiciding grants "I don't need virgins for this". Stand near a Taliban and suicide. The default key to suicide is "K".
Putting a burning NPC out grants "I don't know whether to kiss you or kill you". Peeing on a burning NPC puts him out. Using a gas can, molotov, napalm or spray set an NPC on fire and pee on him. You can find a gas can in the house in front of your trailer.
Playing fetch with your dock using a decapitated head grants "It's not cheating, because it's YOUR dog". Kicking a head while you have a recruited dog makes him run towards it.
Peeing on yourself grants "It's sterile and i like the taste". Looking up and peeing makes you pee on yourself. If you happen to be on fire, do this to put yourself out.
Going on a killing spree grants "John Rambo'd!". When going on a killing spree, The Dude starts counting. After he counts a couple of times you should get this achievement.
Resurrecting a zombie grants "Michonne ain't got nothin' on me". Destroy a zombie's head and slice him in half. Pee on the upper half of the zombie until it resurrects. Zombies are found in the "Apocalypse Weekend" expansion
Setting 30 people on fire grants "OMG, WE'RE HAVING A FIRE...sale". You can set people on fire with a gas can, molotov, napalm or spray.
Bribing your way out of arrest grants "Paid the Piper". If you're getting arrested, you can select the cash in your inventory and offer it to the officer.
Killing 10 dogs while a cat is on your gun grants "Pussy on a Pedestal". When wielding the "Machine gun" or the "Shotgun" using a cat from your inventory mounts it to your gun.
Blowing up 30 cars grants "Rebecca Black's Nightmare". Shoot a car, see what happens.
Having 15 or more cats in your inventory grants "Reddit would be proud." Running normally towards a cat makes it run away. Hold the walk key (default shift) to sneak up on cats. Killing the cat does not let you pick it up.
Destroying 30 zombie heads grants "Rick Grimes 4 Life". The zombies are introduced in the "Apocalypse Weekend" expansion.
Having an NPC puke by pissing in their face grants "R. Kelly's Protege". Peeing on an NPC has a chance of them puking or running away.
Running 30 miles while wielding scissors grants "Running With Scissors". The scissors can be found in the basement of the in game RWS office. Don't trip.
Eating 30 doughnuts while wearing the police officer's uniform grants "Señor Cornballer".
Killing 30 people of each skin color grants "Sheriff Arpaio would be proud". "Please don't think I'm a bigot, I kill races equally..." - The Dude
Getting your sledge hammer stuck in a cow grants "Shiva Blast". This is possible to get during the "Kill Cows" mission in the "Apocalypse Weekend" expansion. Get behind a cow and throw a sledgehammer in its behind.
Putting a cat on your gun grants "Taxidermy with Chuck Testa". When wielding the "Machine gun" or the "Shotgun" using a cat from your inventory mounts it to your gun.
Escaping from the maximum-security jail cell grants "Uncle T-Bag". When you're arrested, you wind up in jail. Throwing matches at the smoke detector can set off the alarm and free you. Get arrested multiple times to upgrade to a more secure cell.
Kicking a door open grants "Well, aren't YOU a badass". (Default "Q")