number is the internal date number.
mode sets the start of the week and the calculation type; 1 = Sunday, 2 = Monday.
January 1, 1995 was a Sunday (mode
value of 1). Both =WEEKNUM("1995-01-01",1)and
in IBM Docs. Even when the week begins on Monday, January 1 (Sunday) is computed to belong to the first week of the year.
Parent topic: Date and time functions