This feast is also called The Entrance of the Theotokos into the Temple in Orthodox tradition and The Presentation of the Blessed Virgin Mary in Catholic tradition. It commemorates the entrance or presentation of Mary as a child at the Temple in Jerusalem by her parents to serve as one of the Temple virgins. It’s celebrated on November 21.
It’s not clear to me exactly where this feast ranks in Catholic tradition, but it’s one of the twelve Great Feasts of the Orthodox liturgical year. (Four of the Great Feasts commemorate the Theotokos; this is the second one. As a note Pascha is not one of the twelve Great Feasts. Rather, it is considered the Feast of Feasts and stands alone and above all other feasts.)
Fr. Thomas Hopko has an essay online about the feast that’s well worth reading. As he notes, a central theme of the feast revolves around Mary entering the Temple to become herself the living Temple of God. As such, her entrance into the Temple celebrates the end of the physical temple in Jerusalem as the dwelling place of God.