The date of the Revelation at Mount Sinai.
My kid knows it occured on the sixth day of Sivan. Chazal, however, weren't quite so sure. In fact, some well known Tanaim argue this question quite strenuously on the pages of BT Yoma (by which I mean Talmud Bavli, Meseches Yooomah.)
IIRC the Rabbis say the Lord appeared in thunder and lightning, with a thick cloud over the mountain, and a very loud trumpet blast on the sixth of Sivan.
Whereas Rabbi Jose (who was not Hispanic) says the great day was on the seventh of Sivan.
Who was right? Who knows? (aside from my kid, I mean)