drop down list value in asp.net

<asp:DropDownList ID="DdlMonths" runat="server">
    <asp:ListItem Enabled="true" Text="Select Month" Value="-1"></asp:ListItem>
    <asp:ListItem Text="January" Value="1"></asp:ListItem>
    <asp:ListItem Text="February" Value="2"></asp:ListItem>
    ....
    <asp:ListItem Text="December" Value="12"></asp:ListItem>
</asp:DropDownList>

You can even use a RequiredFieldValidator which ignore this item, it considers it as unselected.

<asp:RequiredFieldValidator ID="ReqMonth" runat="server" ControlToValidate="DdlMonths"
    InitialValue="-1">
</asp:RequiredFieldValidator>