Calendar control in asp.net

ASP.NET Calendar control has Property for set Today's Date CSS

TodayDayStyle-CssClass

You can apply the Current Date Back Color as

TodayDayStyle-BackColor="Red"

If you want more customization, you can use use DayRender Event

protected void Calendar1_DayRender(object sender, DayRenderEventArgs e)
       {
      // Customize your Calendar
       }

Hope this answer your question.