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first day

noun

Definition of first day 

1 usually capitalized F : sunday —used chiefly by the Friends

2a : a day when the postage stamps of a new issue are first placed on sale especially : one on which recognition of the event is made by special cancellations and often by cachets on mail bearing the stamps

b : first day cover

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First Known Use of first day

1649, in the meaning defined above

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The first known use of first day was in 1649

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