I believe the first day of the year is called Lightday, the term was mentioned in one of the backstory chapters and I suspect it refers to the end of the Weeping and the beginning of the new year.
Also, a week is 5 days long, so there are 500 days in a year, and Brandon has said that the days on Roshar are somewhat shorter then our days. Apparently Roshar is smaller then Earth, as gravity is also only 0.7 of Earth's. So a year on Roshar is only maybe 10 percent longer then ours.