.net - Reselect Date From Calendar Control After Validation Failed?

I have a textbox, a imagebutton and a calendar control inside a user control to pick the date from calendar and set the selected date inside textbox. All work well except the validation. I tried to validate the textbox's value to be a valid date. If it is not valid date, I want to reselect date from calendar. but looks like if valication failed, I could not reselect date if I did not put a correct date inside textbox. Basically validation requires me to fix invalid date first before I try to select a valid date. but I want to reselect when validation failed.

protected void Calendar1_SelectionChanged(object sender, EventArgs e)
{ // Set Date Time value into the TextBox control
TextBox1.Text = Calendar1.SelectedDate.ToString("MM/dd/yyyy");
// Hide the Calendar control after selecting the date
Calendar1.Visible = false;
}
protected void ImgButton1_Click(object sender, EventArgs e)
{
Calendar1.Visible = !Calendar1.Visible;
}