The result indicates the number of the calendar week for a Date.
The WEEKNUM_EXCEL2003 function is designed to calculate week numbers exactly as Microsoft Excel 2003 did. Use the WEEKNUM function for ODF OpenFormula and Excel 2010 compatibility, or ISOWEEKNUM function when you just need ISO 8601 week numbers. In releases prior to LibreOffice 5.1 WEEKNUM_EXCEL2003 was named WEEKNUM_ADD.
Date: the date within the calendar week.
Return type: 1 for week beginning on a Sunday, 2 for week beginning on a Monday.
In which week number does 12.24.2001 fall?
=WEEKNUM_EXCEL2003(DATE(2001;12;24);1) returns 52.