Best practice to check in client side for validation.
You may use this code
<asp:TextBox ID="tbxEventDate" Enabled="true" runat="server" CssClass="datepicker input-xlarge"></asp:TextBox>
<asp:RequiredFieldValidator runat="server" ControlToValidate="tbxEventDate" ID="rfvDate"
ValidationGroup="validate" CssClass="text-error" Text="required" SetFocusOnError="true"></asp:RequiredFieldValidator>