First Day Cover for the wedding of Prince Charles and Lady Diana Spencer, issued 22nd July 1981. A commemorative crown is set into the envelope.

The first day of issue is the day on which a postage stamp, postal card or stamped envelope is officially put on sale, usually in a particular city (usually within the country or territory of the stamp-issuing authority but sometimes from a temporary or permanent foreign or overseas office), though sometimes in several cities simultaneously, nationwide or in a particular region; later the item will usually become available in a wider area. 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.

first day cover, US 1958

(The stamps issued by private local posts can also have first days of issue, as can artistamps.)

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 different ways:

• Stamps may be inadvertently sold or stolen, and cancelled on an envelope or package by unaware postal officials prior to the first day of issue.
• Minor changes, such as a different perforation, may not be noted by officials, and no one knows when they first went on sale. This is also true of some major stamp issues, especially during periods of civil unrest or if government records have been lost.
• Some earlier stamps, especially high values, may not have found any customers using them on the day of issue, or those uses have been lost. EKUs for these may be weeks or even months after the official first day.

The search for EKUs of both old and new stamps is an active area of philately, and new discoveries are regularly announced.

First Day Digital Color Postmark

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.[1] (http://www.usps.com/communications/news/stamps/2004/sr04_080.htm)

US first day postmark