Check following links, these might help you:
Restrict-the-Calendar-selection-within-a-specific-range[
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How to Disable Selected Dates dynamically from the Calendar Control[
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http://forums.asp.net/t/1161833.aspx[
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You can check if following code helps you, its in VB.Net, you can convert it to C# if that is your language:
Private Sub Calendar1_DayRender(sender As Object, e As DayRenderEventArgs)
If e.Day.Date < BeginningOfDateRange
e.Cell.BackColor = System.Drawing.Color.LightGray
e.Day.IsSelectable = False
End If
If e.Day.Date > EndOfDateRange Then
e.Cell.BackColor = System.Drawing.Color.LightGray
e.Day.IsSelectable = False
End If
End Sub