Probably something like this
Option Explicit
Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
Dim myDate As Date
Dim r As Range
Set r = Target.Cells(1, 1)
If Intersect(r, Range("A2:A100")) Is Nothing And Intersect(r, Range("D2:D100")) Is Nothing Then Exit Sub
Set clsCal = New ClsCalendar
FormPicker.Show
myDate = clsCal.SelectedDate
If myDate > 0 Then 'Check to see if it was cancelled
Application.EnableEvents = False
Target.Value = clsCal.SelectedDate
Application.EnableEvents = True
End If
Finally:
Set clsCal = Nothing
Cancel = True
End Sub