If you are using asp calender control then
By using ondayrender property we can highlight particular dates(by changing background/fore color)
in.aspx page
<asp:Calendar ID="Calendar1" runat="server" ondayrender="MyDayRenderer">
in aspx.cs page
<pre lang="cs">protected void MyDayRenderer(object sender, DayRenderEventArgs e)
{
if (e.Day.IsToday)
{
e.Cell.BackColor = System.Drawing.Color.Aqua;
}
if (e.Day.Date == new DateTime(2011,5,1))
{
e.Cell.BackColor = System.Drawing.Color.Beige;
}
}</pre>