because, well, it was SATURDAY!
It became a feast day in the Roman Catholic Church due to the influence of the Waterford-born Franciscan scholar Luke Wadding2 in the early part of the 17th century, and is a holy day of obligation for Roman Catholics in Ireland. The date of the feast is occasionally moved by church authorities when March 17 falls during Holy Week; this happened in 1940 when Saint Patrick’s Day was observed on April 3 in order to avoid it coinciding with Palm Sunday, and happened again in 2008, having been observed on 15 March. March 17 will not fall during Holy Week again until 2160.
