The first day of issue is the day on which a postage stamp, postal card or stamped envelope is put on sale, within the country or territory of the stamp-issuing authority. Sometimes the issue is made from a temporary or permanent foreign or overseas office. 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. 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. (The stamps issued by private local posts can also have first days of issue, as can artist stamps.)

A first day cover is an envelope where the postage stamps have been cancelled on their first day of issue.

Information courtesy of thefreedictionary.com and Wikipedia, February 2007.