Dropdown List selection changed event not firing

Question

I used Dropdowmlist in my Content page.

if i select the 'Day1' from the dropdownlist item

the event is firing.

but if i select the 'Day1' again from the dropdownlist item

the event is not firing.

        <asp:DropDownList ID="ddlSelectDayforTest" AutoPostBack="true" runat="server" OnSelectedIndexChanged="ddlSelectDayforTest_SelectedIndexChanged" Height="16px" >

                        <asp:ListItem Value="0">Select</asp:ListItem>

                        <asp:ListItem Value="1">Day1</asp:ListItem>

                        <asp:ListItem Value="2" Enabled="false">Day2</asp:ListItem>

                        <asp:ListItem Value="3" Enabled="false">Day3</asp:ListItem>

                        <asp:ListItem Value="4" Enabled="false">Day4</asp:ListItem>

                        <asp:ListItem Value="5" Enabled="false">Day5</asp:ListItem>

                         <asp:ListItem Value="6" Enabled="false">Day6</asp:ListItem>

                    </asp:DropDownList>