Option Explicit
Sub Test()
Dim wbData As Workbook
Dim pth As String 'path
Dim prf As String 'préfixe
Dim suf As String 'suffixe
Dim nom As String 'nom
Dim dat As String 'date
pth = "D:\test rapport\"
prf = "Project_Rapport 2020_Situation -"
suf = "_LONG.xlsm"
nom = Dir(pth & prf & "*" & suf)
Do While nom > ""
nom = Right("0" & Replace(Replace(nom, prf, ""), suf, ""), 8)
If Mid(nom, 5, 4) & Mid(nom, 3, 2) & Mid(nom, 1, 2) > dat Then dat = Mid(nom, 5, 4) & Mid(nom, 3, 2) & Mid(nom, 1, 2)
nom = Dir
Loop
Set wbData = Workbooks.Open(pth & prf & Mid(dat, 7, 2) & Mid(dat, 5, 2) & Mid(dat, 1, 4) & suf)
'...
End Sub