Sub test_fim_mois()
Dim i As Long
Dim fin_mois As Date
Application.ScreenUpdating = False
For i = [A65536].End(xlUp).Row To 2 Step -1
fin_mois = DateAdd("d", -1, DateSerial(Year(Cells(i, 1)), Month(Cells(i, 1)) + 1, 1))
Select Case Weekday(fin_mois)
Case 1
Cells(i, 3) = DateSerial(Year(Cells(i, 1)), Month(Cells(i, 1)) + 1, 0) - 2
Case 7
Cells(i, 3) = DateSerial(Year(Cells(i, 1)), Month(Cells(i, 1)) + 1, 0) - 1
Case Else
Cells(i, 3) = DateSerial(Year(Cells(i, 1)), Month(Cells(i, 1)) + 1, 0)
End Select
Next i
Application.ScreenUpdating = True
End Sub