Deuteronomy 14:28 "At the end of three years, you shall bring forth all the tithes of your produce"

(This Mitzvah only apply in Eretz Yisrael.)

All farming in Eretz Yisrael revolves around a seven year Shemitah cycle (See Positive Mitzvah 134).

The second tithe (Mitzvah 128) is given in the first, second, fourth, and fifth years of the cycle.

In the third and sixth years, a different tithe is given.

We are commanded to set aside this tithe for poor and needy people.