Populating Dropdownlist from List collection c#/asp.net

I have several static asp.net dropdown in my aspx page. I like to create a class and list of values instead of having all this in my page, call that class to populate dropdown. What should I do?
 
This is one of my dropdowns:
<asp:DropDownList ID="dob_day" runat="server" class="W58 MR8" > <asp:ListItem value="">Day</asp:ListItem> <asp:ListItem value="1">1</asp:ListItem> <asp:ListItem value="2">2</asp:ListItem> <asp:ListItem value="3">3</asp:ListItem> <asp:ListItem value="4">4</asp:ListItem> <asp ...