Yes, this is happening because if you double-click on a date that has no values, then it's passing a null to the OpenDay function, which is looking for a long. That works fine if you have a value there because it passes the long integer, which is the ID of that calendar item. If it's an empty date, that passes a null.

The way to fix it is to have the OpenDay function just accept a variant so it can take that null in. If it gets a null, then you can just open the form to add a new date. Otherwise, open the record that sends in. I'll fix the template, but here's the fix for now if you want to put it in your database.

Function OpenDay(D As Variant)

    If IsNull(D) Then
        DoCmd.OpenForm "CalendarF", , , , acFormAdd
    Else
        DoCmd.OpenForm "CalendarF", , , "CalendarID=" & D
    End If

End Function

This way, if it gets a null value, it'll open to a new record. Otherwise, it will open that calendar item. Another thing I'd like to add is that if you do double-click on a date that doesn't have any items in it, when it opens up to add, it should open up to add and default that date for you. Give me a few minutes. I'm gonna add that too. All of this is moot because I've got a calendar 2.0 database coming out soon lol.

Okay, what I did was I added another event, which is the on got focus event. Whenever you click on a field, it'll set a temp var noting what the last box you clicked on was, so it knows what date you've selected, and then when you open up the form that adds the new item. The default value reads that temp var.

Add to the MonthlyCalendarF Form Load event where it creates the events:

Me.Controls("Day" & X).OnGotFocus = "=SetLastClickedDate(" & X & ")"

Add to Global Mod:

Public Function SetLastClickedDate(BoxClicked As Long)

    TempVars("LastClickedDate") = Forms!DatePickerF!MonthlyCalendarF.Form.Controls("Date" & BoxClicked).Value

End Function

Set the Default Value property of the DateTime field on the CalendarF to:

=TempVars("LastClickedDate")

And there you go! You got a bug fix and a little cool addition for today.

I uploaded my database to the lessons page. Enjoy!