When a new stamp is issued in the United States, it is offered for sale in (usually) only one particular city, which has significance to that stamp. This is done on one set date, and then throughout the country on the second day. (Some variations of this have evolved recently.) The date of sale in that city is designated as the "Official" First Day of Issue. A special cancel is applied to the new stamp in the official city only on that initial date. This cancel slogan is "First Day of Issue".

Someone may purchase these stamps at the official city and have them canceled on the same day in a different city, which gives an "Unofficial" First Day Cancel.  This has been noted to be quite challenging at times.

A First Day Cover (FDC) is an exclusive presentation of when, where and why the stamp was issued. FDC's are usually on envelopes. However, sometimes the stamp with the First Day postmark can be found on other items, such as on a postcard, souvenir, piece of wood, bark, cloth, or anything that will accept a stamp and postmark.

Frequently, FDC's are on envelopes accompanied by a cachet.

Adapted from First Day Cover Catalogue and Checklist by Michael A. Mellone