public Integer weekOfMonth(Date todaysDate){
        Integer weekCount = 0;
        Integer startWeekResidue = 0;
        Integer endWeekResidue = 0;
        
        //Calculating startWeekResidue
        Date dt = todaysDate.toStartOfMonth().addDays(-1);
        Date dtFirstWeekend = dt.toStartOfWeek().addDays(6);
        startWeekResidue = dt.daysBetween(dtFirstWeekend);
        
        //Calculating endWeekResidue
        Date dtLastWeekend = todaysDate.toStartOfWeek().addDays(-1);
        endWeekResidue = dtLastWeekend.daysBetween(todaysDate);
        
        //Counting the weeks
        weekCount = (todaysDate.day() - (startWeekResidue + endWeekResidue))/7;
        weekCount += (startWeekResidue > 0 ? 1:0)+(endWeekResidue > 0 ? 1:0);
        System.Debug('Week Number: ' + weekCount);
        return weekCount;
}

Example: Pass Parameter: 13Sept2017 – Output: 3