Its origins can be found as we read Exodus 23 and Leviticus 23. On the Jewish Calendar, this year the feast starts at sundown on the 26th, and concludes at sundown on the 27th May. It is exactly 50 days (7 weeks) from Pesach/Passover – hence the name Shavuot, simply translated from the Hebrew as Weeks. It is a time to thank God for his blessing and abundance and to rejoice over the final grain harvest which would see the children of Israel through till the next season.
Additionally, this feast is one of the three chief feasts when God required the people to come up to Jerusalem to give their offerings each year, so it was a time of pilgrimage and sanctification for the whole harvest as well.