"Under the International Organization for Standardization (ISO) standard 8601, a week always begins on a Monday, and ends on a Sunday. The first week of a year is that week which contains the first Thursday of the year, or, equivalently, contains Jan-4.
While this provides some standardization, it can lead to unexpected results - namely that the first few days of a year may not be in week 1 at all. Instead, they will be in week 52 of the preceding year! For example, the year 2000 began on Saturday. Under the ISO standard, weeks always begin on a Monday. In 2000, the first Thursday was Jan-6, so week 1 begins the preceding Monday, or Jan-3. Therefore, the first two days of 2000, Jan-1 and Jan-2, fall into week 52 of 1999.
An ISO week number may be between 1 and 53. Under the ISO standard, week 1 will always have at least 4 days. If 1-Jan falls on a Friday, Saturday, or Sunday, the first few days of the year are defined as being in the last (52nd or 53rd) week of the previous year.
Unlike absolute week numbers, not every year will have a week 53. For example, the year 2000 does not have a week 53. Week 52 begins on Monday, 25-Dec, and ends on Sunday, 31-Dec. But the year 2004 does have a week 53, from Monday, 27-Dec , through Friday, 31-Dec."
The first week of 2005 should start on January 3. The first and second would be part of week 53 of 2004.
"The function WEEKNUM() in the Analysis Toolpack addin calculates the correct week number for a given date, if you are in the U.S. The user defined function shown here will calculate the correct week number depending on the national language settings on your computer."