see the documentation:http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfsystemwebuiwebcontrolscalendarclassdayrendertopic.asp <%@ Page Language="C#" AutoEventWireup="True" %> <html> <head> <script language="C#" runat="server">
void DayRender(Object source, DayRenderEventArgs e) { // Change the background color of the days in the month // to yellow. if (!e.Day.IsOtherMonth && !e.Day.IsWeekend) e.Cell.BackColor=System.Drawing.Color.Yellow; // Add custom text to cell in the Calendar control. if (e.Day.Date.Day == 18) e.Cell.Controls.Add(new LiteralControl("<br>Holiday")); } </script>
<%@ Page Language="C#" AutoEventWireup="True" %>
<html>
<head> <script language="C#" runat="server">
void DayRender(Object source, DayRenderEventArgs e)
{ // Change the background color of the days in the month
// to yellow.
if (!e.Day.IsOtherMonth && !e.Day.IsWeekend)
e.Cell.BackColor=System.Drawing.Color.Yellow; // Add custom text to cell in the Calendar control.
if (e.Day.Date.Day == 18)
e.Cell.Controls.Add(new LiteralControl("<br>Holiday")); } </script>
<form runat="server"> <h3>DayRender Event Example</h3>
<asp:Calendar id="calendar1"
OnDayRender="DayRender"
runat="server" ........>
</asp:Calendar>
</form>
</body>
</html>
http://aspnet.4guysfromrolla.com/articles/100103-1.aspx