“When you have finished paying all the tithe of your produce in the third year, which is the year of tithing, giving it to the Levite, the sojourner, the fatherless, and the widow, so that they may eat within your towns and be filled,
12. When thou hast made an end of tithing. In this passage Moses urgently stimulates them to offer the tithes willingly and abundantly, by placing God, as it were, before their eyes, as if they paid them into his hand: for a solemn protestation is enjoined, in which they condemn themselves as guilty before God, if they have not faithfully paid the tax imposed upon...
Concerning the disposal of their tithe the third year we had the law before, Deu 14:28, Deu 14:29. The second tithe, which in the other two years was to be spent in extraordinaries at the feasts, was to be spent the third year at home, in entertaining the poor.
Commenting on Deuteronomy 26:12-15
When thou hast made an end of tithing all the tithes of thine increase,.... Which, according to Maimonides (k), is to be understood of the feast, in which all tithes are finished, which is the feast of the passover: the third year, which is the year of tithing; that is, the third from every seventh, when the land lay fallow.