Meaning of first-day cover

first'-day cov'er

Pronunciation: (fûrst'dā", -dā'), [key]
— Philately. Philately.
  1. 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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.