Modifiez votre code comme ceci
Sub Mode_Compta_1()
With Sheets("RdV")
If .ComboBox1.Value = "OUI" Then
.Range("B:CS").EntireColumn.Hidden = False
' Si Lundi = Oui
.Range("J:Q,Z:AG,AP:AW,BF:BM,BV:CC,CL:CS").EntireColumn.Hidden = True
.Rows("2:3").Hidden = False
.Rows("3:3").Hidden = True
' Si Lundi = Non
Else
.Range("B:CS").EntireColumn.Hidden = False
.Range("B:Q,Z:AG,AP:AW,BF:BM,BV:CC,CL:CS").EntireColumn.Hidden = True
.Rows("2:3").Hidden = False
.Rows("3:3").Hidden = True
End If
End With
End Sub