Sforno on Deuteronomy 15:19/ספורנו על דברים ט״ו:י״ט

1כל הבכור, after the Torah has listed a number of acts of kindness you are to perform with parts of your harvest in the field and orchard, something called the “tithe for the poor,” as well as an equivalent at the end of the sh’mittah year in money, i.e. the waiving of past due debts, acts of charity, and acts of helping your former servant to establish an economic base for himself, Moses now explains in detail commandments which are acts of gratitude vis a vis G’d, i.e. the giving to G’d or His representative the firstborn males of your livestock, i.e. sheep, goats and cattle. It is incumbent upon the owner of such herds to thank the Lord, for it is He Who is the source of his wealth. This is followed by repetition of the commandments to observe Passover, the commemoration of our obtaining political freedom, as well as the annual date on which the early barley harvest is acknowledged with an appropriate offering, and of Shavuot, the obtaining of our spiritual freedom when we received the Torah in capsule form, i.e. the Ten Commandments. Shavuot also ranks as the primary harvest festival celebrating the cutting of the wheat harvest. Finally, we observe the Sukkot festival signaling that the last of the various harvests, particularly that of the trees has been successfully brought into the barns. On each of these festivals the farmer brings a gift for G’d with him when he makes the pilgrimage to Jerusalem and the Temple. This is spelled out in 16,16. Seeing that the occasion for this is one or all of these three festivals, there is no need for the Torah to mention a specific date as we all know the dates of these festivals. The Torah does mention the season during which these festivals occur, to remind us that they are connected with times of significance to each farmer.