The Feast of Firstfruits Feast of Weeks, Pentecost are the common titles
given to this particular feast. This particular feast is celebrated after seven
weekly sabbaths have past after the keeping of the Lords Passover/Feast of
Unleavened Bread.

Leviticus 23:11, "And he shall wave the sheaf before the Lord, to be
accepted for you: on the morrow after the Sabbaths ( this particular Sabbath
is referring to the last day of the Feast of Unleavened Bread; when  the priest
shall wave the sheaf ). 

Leviticus 23: 15" And ye shall count unto you from the morrow after the
sabbath, from the day that ye brought the sheaf of the wave offering; seven
sabbaths shall be complete:.. The Greek term utilized for this day is called the
Pentecost. Which is Greek that means fiftieth day.

The Apostles had an understanding of this High Holy day and they observed
Pentecost/Feast Of Firstfruit.

 Acts 2:1, "And when the day of Pentecost was fully come, they were all with
one accord in one place". On this particular feast day the twelve disciples
received spiritual power to speak in different known languages (tongues) and
the power to heal and do miracles.

Acts 2:4, "And they were all filled with the Holy Ghost, and began to speak
with other tongues (known languages)".

On this day / Feast of Firstfruits the men of the nation of Israel brought their
Firstfruit of crops/cattle to the priest and he would wave a sheaf or Palm to
bless the crops.

In Christ we have to keep this feast as a Sabbath. This feast will always fall on the
first day of the week after the numbering seven sabbaths. We are commanded to
rehearse the righteous acts. Judges 5:11