how to get the number of the week from any date

hi

how i can get the number of the week from any date ?

for example:

i put   15/10/2013

                                   row month number5|4|3|2|1|30|29[1]12|11|10|9|8|7|6[2]19|18|17|16|[15]|14|13[3]==>> here is the 15/10  and the result is326|25|24|23|22|21|20[4]2|1|31|30|29|28|27[5]