Option Explicit
Private Sub Workbook_Open()
Dim F As Worksheet, Tmp As String
Tmp = Format(Date, "dd")
On Error Resume Next
Set F = Sheets(Tmp)
On Error GoTo 0
If Not F Is Nothing Then
F.Activate
Else
MsgBox "La feuille " & Tmp & " n'est pas présente", 64, "Attention"
End If
End Sub