Autofill weekdays/skip weekends in a month

see NETWORKDAYS function in XL Help, eg

B2: =NETWORKDAYS(EOMONTH(A2,-1)+1,A2,holidays)
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where holidays is a named range containing public holidays to be excluded in addition to weekends - if not required omit.

If you're working the other way around, which in hindsight I think you are, then see WORKDAY function (works along similar lines to above)

A2: =WORKDAY(DATE(2009,12,0),B2,holidays)
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where B2 contains your workday flag - use of named range as before and optional as before