A first day of issue cover or first day cover (FDC) refers to a postage stamp on a cover, postcard, or stamped envelope franked on the first day the stamp is authorized for use within the country or territory of the stamp-issuing authority.
There will usually be a first day of issue postmark, frequently a pictorial cancellation, indicating the city and date where the item was first issued, and "first day of issue" is often used to refer to this postmark. Depending on the policy of the nation issuing the stamp, official first day postmarks may sometimes be applied to mail weeks or months after the date indicated.
Postal authorities may hold a first day ceremony to generate publicity for the new stamp, 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 to the stamp's subject, such as the birthplace of a social movement, or at a stamp show.
Usually first day covers are more valuable than ordinary used covers, postcards, or envelopes with cancelled stamps. Some rare examples of FDCs have been valued at a few thousands of dollars.