Here i have explained how to show calendar control in asp.net.

First of all register install ajax control toolkit.

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

Step 1: Add below HTML Markup.

<asp:ScriptManager ID="ScriptManager1" runat="server" />
<script src="Extension.min.js" type="text/javascript"></script><asp:TextBox ID="txtDate" runat="server" CssClass="disable_past_dates" />
<asp:ImageButton runat="server" ID="imgPopup" ImageUrl="~/Calendar.png" /><cc1:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="txtDate"
PopupButtonID="imgPopup" />

Step 2: Add the below CSS inside head with style tag.

.ajax__calendar_day_disabled
{
    background-color:#ccc !important;
    color:#eee !important;
}

Screenshot

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