If a user selects "Evening" then the RadDate1 is required.  What is the best way to go about this?  

<asp:DropDownList ID="DropDownListTime" AutoPostBack="true" runat="server">
           <asp:ListItem Text="Not Set" Value="0" />
           <asp:ListItem Text="Morning" Value="1" />
           <asp:ListItem Text="Evening" Value="2" />
</asp:DropDownList>  
 
<telerik:RadDatePicker ID="RadDate1" runat="server" >
          <DateInput ID="DateInput1" Font-Size="8pt" runat="server" /> 
</telerik:RadDatePicker> 


 

Use a RequiredFieldValidator for the date control, and set it Enabled="False".  Now handle the SelectedIndexChanged event of the DDL, and if the selectedvalue is 2, enable the validator, otherwise disable it.