When you are trying to not allow users to type anything in the textbox, but only be filled by the calendar extender and then you try to get the selected date from the textbox control it may be empty string if you have set the textbox property to ReadOnly="True".

Its because read only controls are NOT posted back to the server. Workaround for this is the following:

protected void Page_Load(object sender, EventArgs e)
{
TextBox1.Attributes.Add("readonly", "readonly");
}

Hope it helps.