God commanded that an omer of barley be brought and presented in the Temple as an offering on the second day of Passover. This marks the end of the barley harvest. He further commanded that the days be counted and 49 days, or 7 weeks, later, a new offering is to be presented. Exactly 50 days after Passover is the Holiday, Shavuot (Feast of Weeks), also known as the Feast of Harvest. This is the beginning of the wheat harvest. The 50 days between Passover and Shavuot is known as the Counting of the Omer.

It was 50 days after the Passover, the Hebrew's freedom from slavery, that God gave Moses His law on Mount Sinai. It was 50 days after Passover that God sent His Holy Spirit to indwell the disciples in the upper room in Jerusalem. It is customary to spend these 50 days in reflection and meditation to prepare for the receiving of His Word.

And, being assembled together with them,
commanded them that they should not depart from Jerusalem,
but wait for the promise of the Father, which, said He,
you have heard of me. For John truly baptized with water;
but you shall be baptized with the
Holy Ghost not many days hence.

~Acts 1:4-5