A first day cover (FDC) is an envelope whereupon postage stamps have been cancelled on their first day of issue. Depending on the policy of the nation issuing the stamp, official first day postmarks may sometimes be applied to covers weeks or months after the date indicated.
Unofficial first day of issue postmarks can also occur when a stamp collector purchases the stamps in question from a post office in the first day of issue city and then takes them (on that same day) to a post office in another city to have them cancelled, or when stamps are affixed to envelopes that are simply dropped into the mailstream on the first day and receive that day's normal postmark.
(Computer vended postage stamps issued by Neopost had first-day-of-issue ceremonies sponsored by the company, not by an official stamp-issuing entity. Personalised postage stamps of different designs are sometimes also given first-day-of-issue ceremonies and cancellations by the private designer. The stamps issued by private local posts can also have first days of issue, as can artistamps.)
Postal authorities may hold a first day ceremony to generate publicity for the new issue, with postal officials revealing the stamp, and with connected persons in attendance, such as descendants of the person being honored by the stamp. The ceremony may also be held in a location that has a special connection with the stamp's subject, such as the birthplace of a social movement, or at a stamp show.
Event covers, instead of marking the issuance of a stamp, commemorate events both big (say, the launching of a space shuttle) and small (say, the opening of a supermarket). A design on the left side of the envelope (a "cachet") explains the event or anniversary being celebrated. Ideally the stamp or stamps affixed relate to the event. Cancels are obtained either from the location (e.g., Cape Canaveral, Anytown) or, in the case of the United States, from the Postal Service's Cancellation Services unit in Kansas City.
The earliest known use (EKU) of a stamp may or may not be the same as the first day of issue. This can happen in several ways:
The search for EKUs of both old and new stamps is an active area of philately, and new discoveries are regularly announced.
In 2004 the United States Postal Service announced plans to introduce first day digital color postmarks to be used to cancel some first day covers for commemorative stamps in 2005. The postmarks are designed by the artist or art director who designed the stamp itself.
The Digital Color Postmark is still used on select issues. For example, the 41 cent Year of the Rat stamp issued on January 9, 2008 was available in Digital Color Postmark.