Disable dates other than Monday in Ajax CalendarExtender using JavaScript in ASP.Net

Hi DiemHuong,

Refer the below sample code using JavaScript.

<div>
    <asp:ScriptManager ID="ScriptManager1" runat="server" />
    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <asp:CalendarExtender ID="TextBox1_CalendarExtender" runat="server" Enabled="True"
        TargetControlID="TextBox1" FirstDayOfWeek="Monday" OnClientShown="DisableExceptMonday">
    </asp:CalendarExtender>
</div>
<div>
    <script type="text/javascript">
        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;
                    // This is based on FirstDayOfWeek i.e. Monday.
                    // So change the index i.e. 0 based on your FirstDayOfWeek.
                    if (cell.id != sender._id + "_day_" + i + "_" + "0") {
                        cell.style.setProperty("text-decoration", "line-through");
                        cell.parentElement.style.pointerEvents = 'none';
                    }
                }
            }
        }
    </script>
</div>

Screenshot