ASP Textbox validation fire in the page postback

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ASP.NET

I am using Obout calendar control (obout suite for ASP.NET 3.5) to select the date to asp text box. The form has a button and when click the button, form data will write to sql table and will clear the form control. asp required field validator is used for the textbox.

The error is, in the postback, the validation for the text box is firing.
How to stop this validation from firing when coming back to the page on clicking the button.
Code is given below.

<asp:TextBox ID="TextBox1" runat="server" ReadOnly="True"></asp:TextBox>

    <obout:Calendar ID="Calendar1"
        runat="server" DatePickerMode="True" TextBoxId="TextBox1">
    </obout:Calendar>

    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
        ControlToValidate="TextBox1" ErrorMessage="RequiredFieldValidator"></asp:RequiredFieldValidator>
    <br />
    <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />

the button_click event

protected void Button1_Click(object sender, EventArgs e)
    {
        Calendar1.SelectedDate = new DateTime(0);
        TextBox1.Text = "";
    }