replace your code
<asp:ListBox ID="ListBox1" runat="server"
OnSelectedIndexChanged="ListBox1_SelectedIndexChanged" Width="258px"
AutoPostBack="True" ></asp:ListBox>
with
<asp:ListBox ID="ListBox1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="ListBox1_SelectedIndexChanged">
<asp:ListItem>2017-January</asp:ListItem>
<asp:ListItem>2017-February</asp:ListItem>
</asp:ListBox>
and try. this is working for your code behind