If you need to create an exception out of an item, change any of the item's properties. This will automatically mark the item as an exception. For example, if you need to create an exception by moving an item from Monday to Tuesday, change the item's StartTime and EndTime properties and the control will automatically mark the item as exception (there is no need to hide the item). Considering that 'item' is a reference to the Monday item, here is the code to mark the item as exception by moving it to Tuesday:

item.StartTime += TimeSpan.FromDays(1);
item.EndTime += TimeSpan.FromDays(1); 

Meppy