Numbers 29:13 - Matthew Poole's English Annotations on the Holy Bible

Thirteen young bullocks; more sacrifices than at any other feast, partly because this feast was in the close of the year, when it was meet to supply the defects of the year past, and when they had gathered in all their fruits, Deuteronomy 16:13,15 and therefore ought to make the larger returns and acknowledgment to God; partly because it was God's pleasure so to order it, for reasons known to himself, in whose will we ought to acquiesce. And the same reason holds why these sacrifices grew fewer and fewer every day.