Gav,

After duplicating the error, here's what I came up with:

I created a WinApp with MainForm as MdiParent = True, an EventsForm and
CalendarDialog

On MainForm I placed a button with this code:

EventsForm frm = new EventsForm();
if ( frm.CheckDate() == DialogResult.Ok )
{
   frm.MdiParent = this;
   frm.Show();
}

On EventsForm I added the CheckDate() method:

public DialogResult CheckDate()
{
    return new CalendarDialog().ShowDialog();
}

CalendarDialog has 2 buttons, 1 with DialogResult = Cancel, the other
DialogResult = Ok

Things work as expected.

I thought about the "elegance" of the solution, and decided EventsForm is a
class, with exposed methods and properties like any other class, so
justified it that way :)
Or, you can say that a pre-condition for EventsForm to load is that it has a
valid date to load data with.

Ron