When Emperor Constantine I died, Athanasius was allowed to return to his See of Alexandria. Shortly thereafter, however, Constantine's son, the new Roman Emperor Constantius II, renewed the order for Athanasius's banishment in 338. Athanasius went to Rome, where he was under the protection of Constans, the Emperor of the West. During this time, Gregory of Cappadocia was installed as the Patriarch of Alexandria, usurping the absent Athanasius. Athanasius did, however, remain in contact with his people through his annual Festal Letters, in which he also announced on which date Easter would be celebrated that year.
How did the people of Alexandria know when to celebrate Easter?
Festal Letters