Once a year in Scranton Business Park, a man comes around with a small oven and some soft pretzels as well as toppings. One complementary pretzel is given to whomever works in the building and waits in line. It is seen in the episode "Initiation".
Stanley Hudson is especially fond of this day, mostly because it is a break from "a job for which he gets paid too little". Stanley is also the originator for unofficially naming this day "Pretzel Day".
Erin used "Pretzel Day" as an incentive to get Stanley to wake up hoping it will get him out of bed in the episode Tallahassee.
Pretzel Day is on April 26th.