Roman Catechism 18.148

Chapter 18 — Part II: The Sacraments — The Holy Eucharist

Section 148. The Church Commands; The Faithful To Communicate Once A Year — But subsequently, when charity and devotion had grown so cold that the faithful very seldom approached Communion, it was decreed by Pope Fabian, that all should communicate thrice every year, at Christmas, at Easter and at Pentecost. This decree was afterwards confirmed by many Councils, particularly by the first of Agde.