I am using the following formula to decide which month a certain week belongs to:

=CHOOSE(MONTH(DATE(currYear;1;currWeek*7)-WEEKDAY(DATE(currWeek;1;3)));"Jan";"Feb";"Mar";"Apr";"Maj";"Jun";"Jul";"Aug";"Sep";"Okt";"Nov";"Dec")
But unfortunately I must say that I do not understand the formula fully. But I think it chooses month based on in which month the wednesday of the week is. Is that correct?