Code

private void UpdateVisibility()
{
	calendar1.BeginInit();

	foreach (Item item in calendar1.Schedule.Items)
		item.Visible = true;

	foreach (Item item in calendar1.Schedule.Items)
	{
		ItemCollection items = calendar1.Schedule.GetAllItems(
			item.StartTime, item.EndTime);

		if (items.Count > 1)
		{
			foreach (Item a in items)
			{
				if (a == item)
					continue;
				if (!a.Visible)
					continue;

				if (a.Priority > item.Priority)
					item.Visible = false;

				if (a.Priority < item.Priority)
					a.Visible = false;
			}
		}
	}

	calendar1.EndInit();
}