When we see the April dates on the wall calendar, they fall on the correct weekdays for 1940, when the film is set and when it was made. The April dates shown are the 12th, 16th and 23rd and they are correctly shown to be on Friday, Tuesday and Tuesday, respectively. However, when the due date for manager Blom's payment arrives, which is to be May 13th, this is said to be on a Friday, in order for it to be a "Friday the 13th". However, in reality, May 13th, 1940, was a Monday. The actual months in which the 13th fell on a Friday in 1940 were September and December.