A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown [1882] at sacred-texts.com
Thou shalt take of the first of all the fruit of the earth--The Israelites in Canaan, being God's tenants-at-will, were required to give Him tribute in the form of first-fruits and tithes. No Israelite was at liberty to use any productions of his field until he had presented the required offerings. The tribute began to be exigible after the settlement in the promised land, and it was yearly repeated at one of the great feasts (Lev 2:14; Lev 23:10; Lev 23:15; Num 28:26; Deu 16:9). Every master of a family carried it on his shoulders in a little basket of osier, peeled willow, or palm leaves, and brought it to the sanctuary.
thou shalt say . . . A Syrian ready to perish was my father--rather, "a wandering Syrian." The ancestors of the Hebrews were nomad shepherds, either Syrians by birth as Abraham, or by long residence as Jacob. When they were established as a nation in the possession of the promised land, they were indebted to God's unmerited goodness for their distinguished privileges, and in token of gratitude they brought this basket of first-fruits.
thou shalt rejoice--feasting with friends and the Levites, who were invited on such occasions to share in the cheerful festivities that followed oblations (Deu 12:7; Deu 16:10-15).
When thou hast made an end of tithing all the tithes of thine increase the third year--Among the Hebrews there were two tithings. The first was appropriated to the Levites (Num 18:21). The second, being the tenth of what remained, was brought to Jerusalem in kind; or it was converted into money, and the owner, on arriving in the capital, purchased sheep, bread, and oil (Deu 14:22-23). This was done for two consecutive years. But this second tithing was eaten at home, and the third year distributed among the poor of the place (Deu 14:28-29).
thou shalt say before the Lord thy God, I have brought away the hallowed things out of mine house--This was a solemn declaration that nothing which should be devoted to the divine service had been secretly reserved for personal use.
I have not eaten thereof in my mourning--in a season of sorrow, which brought defilement on sacred things; under a pretense of poverty, and grudging to give any away to the poor.
neither . . . for any unclean use--that is, any common purpose, different from what God had appointed and which would have been a desecration of it.
nor given ought thereof for the dead--on any funeral service, or, to an idol, which is a dead thing.