Have you ever seen a $2 with a stamp on it? Ever wonder why someone did that? I'll tell you.
In late 1975, in an effort to reduce costs, the government decided to reintroduce the $2 bill. They hoped that if people started using them they could print half as many $1 bills and save the tax payers a sizable sum of money. Previously, production of the $2 bill was discontinued in 1966 citing "insufficient use". The new $2 bills were officially put into circulation on April 13th, 1976.
As part of the United State's Bicentennial celebration the Post Office was authorized to give out first day cancellations on any $2 that were brought in on April 13th, 1976. April 13th was chosen as it was Thomas Jefferson's birthday whose portrait graced the front of the $2 bill. The only requirement was that you had to affix at least 13 cents worth of postage to it (what it cost to send a first class letter) and have a clerk at the post office hand cancel the stamp.
The bills are super collectible (and affordable) as the cancellation mark denotes the city/state where it was cancelled and the customer could have picked any 13 cent stamp (or combination of lower denomination ones) to affix to the $2 bill for cancellation. The service was only offered to people who physically went down to the Post Office on the 13th which makes them a bit scarcer than they could have been.