Hi Kundan,
Try this...
In DayRender event of calender control write the below code
Protected Sub Calendar1_DayRender(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.DayRenderEventArgs) Handles Calendar1.DayRender
If e.Day.IsToday Then
e.Cell.BackColor = Drawing.Color.Red
End If
End Sub
Thanks