What is an Event Cover:

In philately, a cachet is a printed or stamped design or inscription, other than a cancellation or pre-printed postage, on an envelope (my items are printed on a #6 envelope 6.5"x3.75") to commemorate a postal or philatelic event. There are official and private (independent of postal authorities) cachets. They commemorate everything from the first flight on a particular route, to the Sports Events. Cachets are also frequently made, either by individuals or a government, for first Day of Issue, first day stamp events or "second-day" stamp events. They are often present these event covers.