Frankly I wouldn't attempt to do that. As calendars are volatile and effectively indefinite in scope, such a plan is likely to bring Outlook to a shuddering halt. Instead I would suggest using a macro attached to a ribbon button that will give you the day number of the selected date. The following will do that.
Public Sub DayOfYrCt()
'Graham Mayor - https://www.gmayor.com - Last updated - 07 Feb 2020
Dim oView As Outlook.View
Dim oCalView As Outlook.CalendarView
Dim oExpl As Outlook.Explorer
Dim oFolder As Outlook.Folder
Dim intDayNmbr As Integer
Dim dt As Date
Dim LongDaysInYr As Long
On Error GoTo err_Handler
Set oExpl = Application.ActiveExplorer
Set oFolder = Application.ActiveExplorer.CurrentFolder
Set oView = oExpl.CurrentView
If oView.ViewType = olCalendarView Then
Set oCalView = oExpl.CurrentView
dt = oCalView.SelectedStartTime
intDayNmbr = DateDiff("d", CDate("1/1/" & Year(dt)), dt) + 1
LongDaysInYr = 365 - (Year(dt) Mod 4 = 0) + (Year(dt) Mod 100 = 0) - (Year(dt) Mod 400 = 0)
Beep
MsgBox dt & ": Day " & intDayNmbr & " of " & LongDaysInYr
End If
lbl_Exit:
Set oExpl = Nothing
Set oFolder = Nothing
Set oView = Nothing
Set oCalView = Nothing
Exit Sub
err_Handler:
Beep
MsgBox "Select a date in the calendar before running this process!"
Err.Clear
GoTo lbl_Exit
End Sub