Private Sub Worksheet_Activate()
If Range("B3").Value = "" Then Range("B3").Value = "Saisir date"
End Sub
'Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$B$3" Then
If Range("B3").Value = "" Then Range("B3").Value = "Saisir date"
End If
End Sub