If A1 holds a date, the formula for calculating last Working Day of the month would be
=WORKDAY(EOMONTH(A1,0)+1,-1)
The above formula assumes that your weekends are Saturday and Sunday.
But, if your weekends are different (e.g. in gulf countries), you can use following formula –
=WORKDAY.INTL(EOMONTH(A1,0)+1,-1,"0000110")
Where 0000110 is a 7 character string, 1 represents a weekend and 0 is a working day. First digit is Monday and last digit is Sunday. The above example is for Gulf countries where Friday and Saturday are weekends.
You also have option to give a range which has holidays. In that case, your formula would become
=WORKDAY(EOMONTH(A1,0)+1,-1,D1:D10)
=WORKDAY.INTL(EOMONTH(A1,0)+1,-1,"0000110",D1:D10)
Where range D1:D10 contains the list of holidays.