E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A school-feast, so called from being held on St. Gregorys Day (March 12th). On this day the pupils at one time brought the master all sorts of eatables, and of course it was a dies non, and the master shut his eyes to all sorts of licences. Gregories were not limited to any one country, but were common to all Europe.