Keep value from Session

Halo,
I have a session variable that I want to pass between pages and then use it. When I click Button1 then Data2 is empty. Why it don't keep it's value?
        Dim Data2 as date
Sub Page_Load() If Not Page.IsPostBack Then Data2 = Session("Data") End If End Sub
        Protected sub Button1_Click() Handles Button1.Click
                    label.Text = Data2.AddDays(1)
        end sub    &nbs ...