WEEKNUM calculates the week number of the year for the internal date value.
Number is the internal date number.
Mode sets the start of the week and the calculation type.
1 = Sunday
2 = Monday
=WEEKNUM("1995-01-01";1) returns 1
=WEEKNUM("1995-01-01";2) returns 52. If the week starts on Monday, Sunday belongs to the last week of the previous year.