Hello friend, I didn't find any issue in the code you shared. I tried the same thing and able to get the expected result:
<script type="text/javascript">
function Select_Options() {
var ddl = document.getElementById("<%=ddl_selection_Cet.ClientID%>");
var SelVal = ddl.options[ddl.selectedIndex].value;
switch(SelVal)
{
case "1": alert('Sunday');
break;
case "2": alert('Monday');
break;
case "3": alert('Tuesday');
break;
case "4": alert('Wednesday');
break;
case "5": alert('Thursday');
break;
case "6": alert('Friday');
break;
case "7": alert('Saturday');
break;
}
}
</script>
<asp:DropDownList ID="ddl_selection_Cet" runat="server" onchange="Select_Options()">
<asp:ListItem>-- Select Weekday --</asp:ListItem>
<asp:ListItem>1</asp:ListItem>
<asp:ListItem>2</asp:ListItem>
<asp:ListItem>3</asp:ListItem>
<asp:ListItem>4</asp:ListItem>
<asp:ListItem>5</asp:ListItem>
<asp:ListItem>6</asp:ListItem>
<asp:ListItem>7</asp:ListItem>
</asp:DropDownList>
I would suggest you to set a debugger in JavaScript method and verify if you are getting the selected value.