Is this What you need?
You don't state where you need this to happen so where ever you need to trigger it
I have done it using todays date and +9 to trigger the result - replace the 9 with a 1 when you are happy with the functionality.
Put the 2 functions below in a module and the following line where you need to trigger the check
if isloaded("frmname"

= true and LOM = true then msgbox "Today is the last day of the month"
Function IsLoaded(ByVal strFormName As String) As Integer
' Returns True if the specified form is open in Form view or Datasheet view.
Const conObjStateClosed = 0
Const conDesignView = 0
If SysCmd(acSysCmdGetObjectState, acForm, strFormName) <> conObjStateClosed Then
If Forms(strFormName).CurrentView <> conDesignView Then
IsLoaded = True
End If
End If
End Function
Function LOM as Boolean
Dim MonthNum As Integer
Dim checkMonthNum As Integer
LOM = false
MonthNum = DatePart("m", Date)
checkMonthNum = DatePart("m", Date + 9)
If checkMonthNum - MonthNum = 1 Then LOM = True
end function
regards
jo