try
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="MvcApplication2.WebForm1" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script>
function DisableExceptMonday(sender,args) {
for (var i = 0; i < 6; i++) {
var row = sender._days.children[0].childNodes[1].children[i];
for (var j = 0; j < 7; j++) {
var cell = row.children[j].firstChild;
if (cell.id != sender._id + "_day_" + i + "_" + "0") {
console.log(cell.id)
cell.parentElement.className = 'ajax__calendar_invalid'
}
}
}
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<ajaxToolkit:CalendarExtender ID="TextBox1_CalendarExtender" runat="server"
Enabled="True" TargetControlID="TextBox1" FirstDayOfWeek="Monday" OnClientShown="DisableExceptMonday">
</ajaxToolkit:CalendarExtender>
</form>
</body>
</html>