Once a year in Scranton Business Park, a man comes around with a small oven and some soft pretzels as well as toppings. One complimentary pretzel is given to whoever 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.
While National Pretzel Day is celebrated on April 26th, the specific date of the annual pretzel day celebrated by Scranton Business Park is not known, but must be between September 1st (the date of the preceeding episode) and Diwali (celebrated in late October or early November). A perfect candidate date to commemorate the event would be the origianl air date of this episode, October 19th.