Adding an Event on Specific Date in Calendar control

DayRender event is raised while the Calendar control is being rendered, you cannot add a control that can also raise an event, such as LinkButton. You can only add static controls, such as System.Web.UI.LiteralControl, Label, Image, and HyperLink.

Add Calendar Control on to the page:Calendar.aspx:

<asp:Calendar ID="Calendar1" runat="server">
   </asp:Calendar>

Add Day Render Event:

protected void Page_Load(object sender, EventArgs e)
 {
     Calendar1.DayRender += new DayRenderEventHandler(Calendar1_DayRender);
 }

 void Calendar1_DayRender(object sender, DayRenderEventArgs e)
 {
     if (e.Day.Date.Day == 8)
     {
         e.Cell.Controls.Add(new LiteralControl("<br/>Holiday"));
     }
 }

Output: