Bernhar when I read Your first post more carefully I realise that You want to know the first day of the week of some date not the number of the day of the week:
Try this (not tested):

 FUNCTION FirstDay(dDate)

 LOCAL ret_val
 IF DOW(dDate) == 1
    ret_val = dDate - 6             && To make it Mondey not Sunday
 ELSE
   ret_val = dDate - DOW(dDate) + 2 && To make it Mondey not Sunday
 ENDIF

RETURN CDOW(ret_val) + " "+ DTOC(ret_val)