Christmas ELY cathedral 1189-89 First day cover fdc b
More info here
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.
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.
US first day postmarkEvent 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:
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 postal 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.
Some stamps may not have had an officially designated first day of issue and instead were simply placed on sale whenever the stamps were needed.
The search for EKUs of both old and new stamps is an active area of philately, and new discoveries are regularly announced.
Condition is V Good - excellent (as per image)
may have slight knocks or some discoloration, etc