It isn't. The norm is defined by ISO 8601.
There is a lot of "fun" into date calculations.
The rule is to counts the number of Thursdays on a year to get the correct number of weeks.
What bothers me more is that here it is cultural that weeks start on Sunday, but ISO 8601 says they start on Monday. In Portuguese, the word for Monday can also be translated as "Second", as in "Second day". How to say that the second day is in fact the first? :-)
