The WEEK function returns an integer between 1 and 54 that represents the week of the year. The week starts with Sunday, and January 1 is always in the first week.
http://www.ibm.com/support/knowledgecenter/ssw_ibm_i_71/db2/rbafzscaweek.htm?lang=it#d582397e63
>>-WEEK--(--expression--)--------------------------------------><
- expression
- An expression that returns a value of one of the following built-in data types: a date, a timestamp, a character string, or a graphic string.
If expression is a character or graphic string, it must not be a CLOB or DBCLOB, and its value must be a valid string representation of a date or timestamp.
The result of the function is a large integer. If the argument can be null, the result can be null; if the argument is null, the result is the null value.