Question
What is the source of the custom of celebrating on the Friday night after a boy is born?
Answer
It's an Ashkenazi custom, mentioned by the Rema in Yoreh Deah 265:12. It's done on Friday night because people are available then. Usually the neighbors are invited, and it's customary to serve chickpeas; the custom of eating chickpeas is mentioned in the (German) Yalkut Minhagim. It has been suggested (see Taz, Yoreh Deah 265:13) that the newborn boy is sad because he has forgotten all the Torah he studied in Heaven; note that chickpeas are mourners' food.