If it was a Friday on 1 January 2016, what was the day of the week on 31 December 2016?

2016 is a leap year with 366 days. 366 = 52 weeks + 2 odd days. Starting from Friday, adding 2 days gives Sunday. But since 31 December is the 366th day, counting from Jan 1 as day 1, there are 365 days after Jan 1, giving 365 mod 7 = 1 odd day, so Friday + 1 = Saturday.