First Born

From When a Jew Celebrates

Redeeming the firstborn is another traditional ceremony marked for some babies – firstborn sons. (This refers only to the firstborn of the mother; the father might have other sons if he was previously married.) This ceremony goes back to the Torah (/ˈtɔːrə, ˈtoʊrə/; Biblical Hebrew: תּוֹרָה‎ Tōrā, “Instruction”, “Teaching” or “Law”) The first five books of the Bible, also called “the books of Moses” come from an archery term meaning to shoot. It is called Pidyon ha-Ben (Hebrew: פדיון הבן) or redemption of the first-born son is a mitzvah in Judaism whereby a Jewish firstborn son is “redeemed” by use of silver coins. Interpretations differ in what the firstborn son is to be redeemed from, ranging from being redeemed from their firstborn status, which was stigmatized after the Ten Plagues, or their obligation to serve as a priest., redemption of the firstborn.
Since all creatures and things come from God, the Bible says that all firstborn things and creatures should be shared with God in thanks.

So the first fruits, the first grains, and the first of the flock were brought to the Temple and given to God. It was supposed to be the same with the firstborn sons. They too were to be brought to the Temple to serve God. However, it became the custom for the father to redeem his son from the Temple service by a gift of money. The gift was five shekels.

This ancient custom is continued today by many families. On the thirteenth day after the birth of the mother’s son, a Kohen A descendant of the ancient priestly families is invited to the house – along with all the relatives, of course, for the party that follows.
The parents give the Kohen A descendant of the ancient priestly families the equivalent of five shekels, usually five dollars, and the child is redeemed from special service to God. The Kohen A descendant of the ancient priestly families usually gives the money to charity.