[SOLVED] Return month for weeknumber

Hi. I have a sheet for registering work hours on projects. It is organized in months and weeks, and I want to be able to return the correct month for each weeknumber.

The rules are:

A week starts monday and ends sunday.
If a week has days in two months, it should belong to the month with 3 or more workdays (monday - friday) in it.

ex. week 18 in 2013 should return May.

How can i do this?

cheers

Hawken