Make a joyful noise unto the LORD, all ye lands. Serve the LORD with gladness: come before his presence with singing. Know ye that the LORD he is God: it is he that hath made us, and not we ourselves; we are his people, and the sheep of his pasture. ~ Psalm 100:1-3

Moses instructs the people about the Feast of Tabernacles, the 3rd feast that males over 20 will be required to attend each year when they enter the Promised land.