Not all saints are celebrated on their day of death. For example, St. John Paul II is celebrated on October 22, the day of his papal inauguration.
However, the vast majority of saints are celebrated on their death date, which is also considered their "birthday" into Heaven.
This is the case for St. Patrick, as tradition has passed down over the centuries the date of March 17 as the day he died.
The Catholic Encyclopedia describes the scene of his death:
At Saul (Sabhall), St. Patrick received the summons to his reward on March 17, 493. St. Tassach administered the last sacraments to him. His remains were wrapped in the shroud woven by St. Brigid's own hands. The bishops and clergy and faithful people from all parts crowded around his remains to pay due honour to the Father of their Faith. Some of the ancient Lives record that for several days the light of heaven shone around his bier. His remains were interred at the chieftan's Dun or Fort two miles from Saul, where in after times arose the cathedral of Down.
A 19th-century book, The Story of Saint Patrick, offers a similar account of his death:
Patrick was now an old man how old there are no means of exactly determining. It is reported that he passed several of his latest years in Armagh and Saul, always however bearing on his heart the concerns of the church at large in Ireland, for whose establishment and progress he had so long and faithfully labored.
Furthermore, "the words [of] some imprudent, gushing admirer [wrote] at the close of Patrick's Confession, 'On the 17th of March Patrick was translated to heaven.'"
Ever since, March 17 has been one of the most popular saint feast days in the world.
