a cover marked so as to indicate that it was mailed on the first day of issue of the stamp it bears and from one of the cities at which the stamp was issued on that day.
Origin of first-day cover
First recorded in 1935–40
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philately a cover, usually an envelope, postmarked on the first day of the issue of its stamps
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012