Two ways of doing it.
<asp:Calendar ID="planning" runat="server" SelectedDate="<%# DateTime.Now %>"></asp:Calendar>
protected void Page_Load(object sender, EventArgs e)
{
BindCalendar();
}
private void BindCalendar()
{
planning.SelectedDate = DateTime.Today;
}
Altough, I strongly recommend to do it from a BindMyStuff way. Single entry point easier to debug. But since you seems to know your game, you're all set.