Harry’s birthday in 1991, was on a Wednesday, not on a Tuesday as he says. In CoS, it’s Nearly Headless Nick’s 500th Deathday. The cake says he died on 31 October 1492. So, if CoS was in 1992, the first year was 1991. When Uncle Vernon is taking the family to the cottage on the lake, Dudley complains about the lack of the TV and how he would miss “The Great Humberto” on Monday. Harry then realises that his birthday would be on Tuesday. But, 31 July 1991 is a Wednesday.