getting macro dynamic

Try this:

last = Range("A3").End(xlDown).row
With Range("M3:M" & last).Validation
     .Delete
     .Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:= _
      xlBetween, Formula1:="Monday,Tuesday,Wednesday,Thursday,Friday,Saturday,Sunday"
      .IgnoreBlank = True
     .InCellDropdown = True
     .InputTitle = ""
     .ErrorTitle = ""
     .InputMessage = ""
     .ErrorMessage = ""
     .ShowInput = True
     .ShowError = True
End With