The Feast of Sabbatical Year: Enactments Respecting: Release of all Hebrew Servants
Source: TTT
Exodus 21:2
If you buy a Hebrew servant, he is to serve you for six years. But in the seventh year, he shall go free without paying anything.
Deuteronomy 15:12
If a fellow Hebrew, a man or a woman, is sold to you and serves you six years, then in the seventh year you must set him free.