"Three times a year, all your males shall appear"Deuteronomy 16:16.

Three times a year [on the festivals of Passover, Shavuot and Sukkot] all men are obligated to go to the Holy Temple, to be "seen" by G‑d. Young boys that have the ability to make their trek on their own should be brought along. Once in the Temple, every man is commanded to bring a Burnt Offering, known as an Olat Re'iyah.

This is one of the three mitzvot associated with the festivals; the other two are offering the Festival Offering and Rejoicing.