מִקְצֵ֣ה ׀ שָׁלֹ֣שׁ שָׁנִ֗ים תּוֹצִיא֙ אֶת־כָּל־מַעְשַׂר֙ תְּבוּאָ֣תְךָ֔ בַּשָּׁנָ֖ה הַהִ֑וא וְהִנַּחְתָּ֖ בִּשְׁעָרֶֽיךָ׃
At the end of every three years, even in the same year, thou shalt bring forth all the tithe of thine increase, and shall lay it up within thy gates.
Read kabbalistic (mystical) interpretation of Deuteronomy 14:28: and classical mystical sources.