Meaning
an adjective denoting "fifieth," is used as a noun, with "day" understood, i.e., the "fifieth" day after the Passover, counting from the second day of the Feast, Act 2:1, Act 20:16, 1Co 16:8. For the Divine instructions of Israel see Exo 23:16, Exo 34:22, Lev 1:23-21, Num 1:28-31, Deu 1:16-11.