on Feast Days

Athanasius Circular Letter par 4 (296-373)
And these things were done even during the holy season of Lent[8], about the time of Easter; a time when the brethren were keeping fast
Athansasius Letter 1 par 10 (296-373 ad)
And when the first day of the holy week dawns and rises upon us, on the eleventh day. of the same month (April 6), from which again we count all the seven weeks one by one, let us keep feast on the holy day of Pentecost--on that which was at one time to the Jews, typically, the feast of weeks