Albert Barnes Commentary Exodus 23:19

"The first of the first-fruits of thy ground thou shalt bring into the house of Jehovah thy God. Thou shalt not boil a kid in it mother`s milk." — Exodus 23:19 (ASV)

The first of the firstfruits of thy land - The “best,” or “chief” of the firstfruits, that is, the two wave loaves described (Leviticus 23:17). As the preceding precept appears to refer to the Passover, so it is likely that this refers to Pentecost. They are called in Leviticus, the firstfruits unto the LORD; and it is reasonable that they should here be designated the “chief” of the firstfruits. If, with some, we suppose the precept to relate to the offerings of firstfruits in general, the command is a repetition of (Exodus 22:29).

Thou shalt not seethe a kid in his mother’s milk - This precept is repeated. See the marginal references. If we connect the first of the two preceding precepts with the Passover, and the second with Pentecost, it seems reasonable to connect this with the Feast of Tabernacles. The only explanation that accords with this connection is one referring to a superstitious custom connected with the harvest. In this custom, a kid was boiled in its mother’s milk to propitiate the deities in some way, and the milk was then sprinkled on the fruit trees, fields, and gardens as a charm to improve the crops of the coming year.

Others take it to be a prohibition of a custom of great antiquity among the Arabs: preparing an unwholesome kind of food by stewing a kid in milk with the addition of certain stimulating ingredients. Still others take it in connection with the prohibitions to slaughter a cow and a calf, or a ewe and her lamb, on the same day (Leviticus 22:28), or to take a bird along with her young in the nest (Deuteronomy 22:6). It is thus understood as a protest against cruelty and violating the order of nature.