Adding Suffix to a date variable

Hello, i've recently learned VBA and really enjoying it. I've managed to automate a large proportion of my job with considerably more accuracy than when i was doing it manually, my manager doesn't know yet though so we'll keep that on the down low for a while. Anyway i've basically got the core part of my spreadsheet working well but am having trouble with adding suffix's (st, nd, rd, th) to a date function. The code i've put together can be seen below. Basically when i add a new sheet every week it copies the old one and corrects all the equations, which works fine, and then it adds week commencing and that Monday's date to a cell at the top, which also works fine. But no matter what I try I only get 'st' after the day number and this is the part I have the issue with. Can anyone have a go at correcting my code? Also, if you could explain why my code was going wrong that would be really helpful - always aiming to learn from my mistakes.

Dim dteMonday As Date
    dteMonday = IIf(Weekday(Date) = 1, Date - 6, _
    Date - (Weekday(Date) - 2))
    
    If Day(dteMonday) = 1 Or 21 Or 31 Then
        n = "st"
    ElseIf Day(dteMonday) = 2 Or 22 Then
        n = "nd"
    ElseIf Day(dteMonday) = 3 Or 23 Then
        n = "rd"
    Else
        n = "th"
    End If
    
    Cells(1, 7).Value = "Week Commencing " & Format(dteMonday, "mmmm d") & n

Thanks Andrew