Exodus 23:16-17

Also [you shall observe] the Feast of Harvest (Weeks, Pentecost, or First Fruits), acknowledging the first fruits of your labor, of what you sow in the field. And [third] the Feast of Ingathering (Booths or Tabernacles) at the end of the year when you gather in [the fruit of] your labors from the field. Three times a year all your males shall appear before the Lord GOD.