That, if you aren’t sure, is the date upon which Luther celebrated his first Mass. Of that occasion Luther remarked
“When I was about to hold my first mass, my father sent twenty gulden for food and came with twenty persons, all of whom he put up. Somebody said to him, ‘You must have a good friend here that you should come to visit him with such a large company,’ etc.
“When at length I stood before the altar and was to consecrate, I was so terrified by the words aeterno vivo veto Deo that I thought of running away from the altar and said to my prior, ‘Reverend Father, I’m afraid I must leave the altar.’ He shouted to me, ‘Go ahead, faster, faster!’
“So terrified was I by those words! Already I had forebodings that something was wrong, but God didn’t give me an understanding of this until later.”