The 1st of Sypheros, 299 YK, was the date that Tira Miron and her party confronted Bel Shalor, resulting in Tira Miron sacrificing herself to re-bind the overlord. The faithful gather on this date to remember the story of Tira Miron, of the suffering inflicted upon Thrane, and the sacrifices made by Tira Miron and all templars since.
Many of the faithful will spend the day finding ways to contribute to their communities, to help those less fortunate than themselves as a way of giving thanks to those whose sacrifices have allowed all of us to live our lives (relatively) unbothered by supernatural evils.