BUSDAYMONTHSTART

BUSDAYMONTHEND(dtDateValue)
Returns the first business day in a particular month and year.

REMARKS
* You can use the built-in EOMONTH function to return the date serial number of the last day of a month before or after a date.
* You can use the built-in WORKDAY.INTL function to return the date serial number that is a given number of working days before or after a date.
* You can use the user defined BUSDAYMONTHEND function to
* You can use the user defined BUSDAYNEXT function to
* You can use the user defined BUSDAYPREVIOUS function to
* You can use the user defined BUSDAYYEARSTART function to
* You can use the user defined BUSDAYYEAREND function to
* You can use the user defined BUSDAYNTH function to
* You can use the user defined BUSDAYSADD function to
* You can use the user defined BUSDAYDIFF function to

Public Function BUSDAYMONTHSTART( _ 
         ByVal dtDateValue As Date) _ 
         As Date 

Dim dtEndOfPreviousMonth As Date 

   dtEndOfPreviousMonth = Application.WorksheetFunction.Eomonth(dtDateValue, -1) 

   BUSDAYMONTHSTART = Application.WorksheetFunction.WorkDay_Intl(dtEndOfPreviousMonth, 1) 
End Function 

For instructions on how to add this function to a workbook refer to the page under Inserting Functions