First Day Covers refer to envelope on which a stamp is placed and cancelled on the first day of issue of that stamp. Likewsie it is an an envelope with a new stamp and cancellation showing the date the stamp was issued.
Typically a cachet is applied to the left side of the envelope describing the stamp's issue.
Ref: 123788/2006-09-19
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Stamp (Stamp can be a rubber, wood, or even potato. ..)