It is established knowledge that HP Part 4 (Goblet of Fire) takes place between July 1994 and July 1995 (else Harry's birthdate would not fit into the sequence of school years).

When the Beauxbatons and Durmstrangs arrive on 30 October 1994 for the Triwizard Tournament, Mrs. Rowling repeatedly refers to this day as being a Friday (and Halloween, the day after, a Saturday).

In reality, 30 October 1994 was a Sunday, and Halloween (31 October) a Monday.