For shorthand we use "through" the last full day of the event, but our world event system switches things off at 3:00 AM Eastern. Essentially Midnight over in California. There's no perfect way to do it, but when we say "through" a day, it usually means until 3:00 AM Eastern on the next day. We also formally use the 3:00 AM date in various places, particularly the LOTRO Event Schedule.