As the name implies, first day of issue covers are envelopes postmarked the same day the stamp was issued. Collecting first day covers has been an active subsection of the greater philatelic hobby for more than 100 years. In a culture as obsessed with minute variations as ours, this is perhaps to be expected. If you’re considering expanding your collection or altering your focus to include first day covers, there’s a few things you should know before you begin:

Many US stamps have a designated first day of issue city, where stamps are released often on a relevant anniversary. For example, a stamp commemorating the Louisiana Purchase was released in New Orleans on the 200th anniversary of the event.
First day of issue covers are often released with a special graphic known as a cachet. This graphic, which has no standard size or color requirements, complements the stamp and adds to the cover’s collectable value. Covers can be released with multiple cachets, and many collectors also design their own.
First day of issue (FDI) and earliest known use (EKU) dates are not always the same. Production issues, minor changes and other factors can lead to stamps being delayed or put into circulation before their official first day.