הלכה על שמות 23:14

The commandment of celebration on the festivals: To celebrate on the festivals; and that is that we have been commanded to come up for the festival to the Temple three times a year - and they are [before] Pesach, Shavouot and Sukkot, in order to celebrate there, as it is stated (Exodus 23:14), "Three festivals shall you celebrate for Me in the year." And the matter of celebration is that we should go up there with a sacrifice and we slaughter it as peace-offerings (shelamim) in honor of the festival. And this commandment is repeated twice in the Torah. And they, may their memory be blessed, said in Tractate Chagigah 6b [that] three commandments was Israel commanded on the holiday - celebration, being seen and joy.