I assume you mean to add data to a Day on the calendar? If that is the
case, check out the DayRender event. You can add code within to manipulate
the text of each day. The following is a sample (C#) from VS2003 Help:
// 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"));