Unable to find control in calendar

Hi, I am trying to find the control for a dynamic text box which is created in side a calendar but it 's always give me error. Please help. My dead line for this project is comming. Thanks

Here is my code:

Public Class WebForm1

Inherits System.Web.UI.Page

Protected WithEvents Calendar1 As System.Web.UI.WebControls.Calendar

Public dynamicTb As TextBox

Protected WithEvents Button1 As System.Web.UI.WebControls.Button

Protected WithEvents Label1 As System.Web.UI.WebControls.Label

Dim i As Integer

Public Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

End Sub

Public Sub Calendar1_DayRender(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.DayRenderEventArgs) Handles Calendar1.DayRender

   i = i + 1
   dynamicTb = New TextBox()
   dynamicTb.ID = i
   dynamicTb.Text = "hello " & i
   e.Cell.Controls.Add(dynamicTb)

End Sub

Public Sub Calendar1_SelectionChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles Calendar1.SelectionChanged

   Dim tb1 As TextBox
   tb1 = CType(Me.FindControl(1), TextBox) //this line give me error????   Please help

End Sub

End Class