ISDATELAST

Returns whether a date is the last of a week, month or year.
For instructions on how to add a function to a workbook refer to the page under Inserting Functions

ISDATELAST_OFAMONTH

Returns whether a date is the last day of the month.

Public Function ISDATELAST_OFAMONTH( _ 
Optional ByVal dtDateValue As Date) As Boolean 

Dim iDay As Integer 
Dim iMonth As Integer 
Dim iYear As Integer 
Dim iNoOfDays As Integer 
    Application.Volatile 
    
    If VBA.IsMissing(dtDateValue) Then 
        dtDateValue = VBA.Date 
    End If 

    iDay = VBA.Day(dtDateValue) 
    iMonth = VBA.Month(dtDateValue) 
    iYear = VBA.Year(dtDateValue) 
    iNoOfDays = VBA.Day(VBA.DateSerial(iYear, iMonth + 1, 1) - 1) 
     
    If (iDay = iNoOfDays) Then ISDATELAST_OFAMONTH = True 
    If (iDay < iNoOfDays) Then ISDATELAST_OFAMONTH = False 
End Function 

ISDATELAST_OFAWEEK

Returns whether a date is the last day of the week.

Public Function ISDATE_LASTOFAWEEK( _ 
Optional ByVal dtDateValue As Date) As Boolean 

    Application.Volatile 
    
    ISDATELAST_OFAWEEK = dtDateValue - VBA.Weekday(dtDateValue, vbUseSystemDayOfWeek) + 7 
End Function 

ISDATELAST_OFAYEAR

Returns whether a date is the last day of the year.