Deuteronomy 15:20

Thou shalt eat it before the Lord thy God year by year. — This connects the eating of the firstlings with the “second” tithe (Deuteronomy 14:23), There is some difficulty in understanding the exact relation between this precept and that which assigns the firstlings to the priests (Numbers 18:15) with the first tithe. The practical solution is to be sought in the practice of the Jews. One suggestion is (that of Rashi), that “thou shalt eat” in this place refers to the priest; another is, that the firstlings without blemish were for the priest; those that were not fit for sacrifice were for the household of the owner. But it is perfectly conceivable that there was a collection of firstlings at one time of the year for the first tithe, and these were given to the priests. At the time of the collection of the second tithe, there might, and generally would, be other firstlings born since, and these, with the second tithe, would be disposed of in the manner indicated in these verses. And this, upon the whole, seems the most probable explanation. If two tithes were a regular institution, they must have been regularly collected at fixed times. And there might easily be firstlings in both of them; in fact, there almost certainly would be. At any rate, no contradiction can be maintained as between laws which were both observed in practice by the Jews. It appears from the Talmud, that tithes and offerings might be presented, more or less, at any of the three great feasts. They would not all be presented at one time. The tithes and first-fruits in some cases were liable to be delayed. The rule was, that everything due for three years last past must be cleared out of the establishment, and paid over to the proper authorities at the Feast of the Passover in the fourth and eighth years reckoned by the Sabbatical system. (See Deuteronomy 26:12, for more on this head.)