How to determine the last business day in a given month?

var holidays = new List<DateTime>{/* list of observed holidays */};
DateTime lastBusinessDay = new DateTime();
var i = DateTime.DaysInMonth(month, year);
while (i > 0)
{
  var dtCurrent = new DateTime(year, month, i);
  if(dtCurrent.DayofWeek < DayOfWeek.Saturday && dtCurrent.DayOfWeek >Sunday &&
   !holidays.Contains(dtCurrent))
    {
      lastBusinessDay = dtCurrent;
      i = 0;
    }
    else
    {
      i = i.AddDays(-1);
    }
}

Function GetLastBusinessDay(ByVal Year As Integer, ByVal Month As Integer) As DateTime
    Dim LastOfMonth As DateTime
    Dim LastBusinessDay As DateTime

    LastOfMonth = New DateTime(Year, Month, DateTime.DaysInMonth(Year, Month))

    If LastOfMonth.DayOfWeek = DayOfWeek.Sunday Then
        LastBusinessDay = LastOfMonth.AddDays(-2)
    ElseIf LastOfMonth.DayOfWeek = DayOfWeek.Saturday Then
        LastBusinessDay = LastOfMonth.AddDays(-1)
    Else
        LastBusinessDay = LastOfMonth
    End If

    Return LastBusinessDay

End Function

Day day = getLastDayOfMonth
int days = getDaysInMonth
for i = days to 0
  if day is weekday
    if day is not holiday
      return day
    end if
  end if
  day = prevDay
  days--
end for

throw exception because no business day was found in the month

Function GetLastDay(ByVal month As Int32, ByVal year As Int32) As Date
        Dim D As New Date(year, month, Date.DaysInMonth(year, month))
        For i As Integer = 0 To Date.DaysInMonth(year, month)
            Select Case D.AddDays(-i).DayOfWeek
                Case DayOfWeek.Saturday, DayOfWeek.Sunday 'Not a weekday
                Case Else  'Is a weekday.  Flag as first weekday found
                    Return D.AddDays(-i)
                    'you can add other code here which could also do a check for holidays or ther stuff since you have a proper date value to look at in the loop
            End Select
        Next
End Function