As a game design and programming student my assumption is that Based on how the banners and other things seem to interact with the game and how they changed for banner as well as this festival, along with how they've described the director system they use to control everything, I would say that all these events are either pre designed as things they can toggle on and off via the director or more likely they re editable modules that they can plug into the director to alter the tower. All in all its probably something that does take some work and time but nothing so intensive that its noticeably slow other kinds of content. Specially considering they have an increasingly massive team and that a realistically small portion of them could manage a hand full of short quests and a handful of items for a modularly programmed event structure without strenuous work or effort.