Pentecost

i.e., "fiftieth", found only in the New Testament (Acts 2:1;
20:16; 1 Cor. 16:8). The festival so named is first spoken of in
Ex. 23:16 as "the feast of harvest," and again in Ex. 34:22 as
"the day of the firstfruits" (Num. 28:26). From the sixteenth of
the month of Nisan (the second day of the Passover), seven
complete weeks, i.e., forty-nine days, were to be reckoned, and
this feast was held on the fiftieth day. The manner in which it
was to be kept is described in Lev. 23:15-19; Num. 28:27-29.
Besides the sacrifices prescribed for the occasion, every one
was to bring to the Lord his "tribute of a free-will offering"
(Deut. 16:9-11). The purpose of this feast was to commemorate
the completion of the grain harvest. Its distinguishing feature
was the offering of "two leavened loaves" made from the new corn
of the completed harvest, which, with two lambs, were waved
before the Lord as a thank offering.

The day of Pentecost is noted in the Christian Church as the
day on which the Spirit descended upon the apostles, and on
which, under Peter's preaching, so many thousands were converted
in Jerusalem (Acts 2).