Place the select parameter inside the SqlDataSource and it will work:

<asp:SqlDataSource ID="MOCSetsDataSource" runat="server"
        ConnectionString="<%$ ConnectionStrings:ConString %>"
        SelectCommand="SELECT * FROM [mocSetsTbl] LEFT JOIN [periodTbl] ON [mocSetsTbl].[p_id] = [periodTbl].[p_id] LEFT JOIN [setsTbl] ON [mocSetsTbl].[s_id] = [setsTbl].[s_id] Where [mocSetsTbl].[p_id] = @month ">
<SelectParameters>
<asp:Parameter Name="month" DefaultValue="1" />
</SelectParameters>
</asp:SqlDataSource>