You can do it in the .aspx file:
<DayPilot:DayPilotScheduler ID="DayPilotScheduler1" runat="server"
....
>
<Separators>
<DayPilot:Separator Color="green" Location="02/01/2008 12:00:00" />
<DayPilot:Separator Color="green" Location="02/03/2008 12:00:00" />
<DayPilot:Separator Color="red" Location="02/02/2008 00:00:00" />
</Separators>
</DayPilot:DayPilotScheduler>
You can also add it in the code behind:
protected void Page_Load(object sender, EventArgs e)
{
DayPilotScheduler1.Separators.Add(DateTime.Today, Color.Red);
}