The common noun for the proper noun Saint Patrick's Day is holiday.

it is not official a holiday

Saint Patrick's was merely a saint's feast day, which happens all the time. But Irish people celebrated elaborately, since St. Patrick was Irish. Eventually, it became a well-known holiday.

In 1903, Saint Patrick's Day became an official public holiday in Ireland Saint Patrick's Day is a religious holiday on the 17th of March. It is named after Saint Patrick the most commonly recognized of the patron saints of Ireland Wadding In the early 1600s. Saint Patrick's Day thus became a holy day of obligation for Roman Catholics in Ireland

..Uhm, it doesn't DO anything, it's a holiday.