The best thing ever. The staff did a great job last year creating the event last year. It was the first one to take place since the Zetaboards (ancient TBT from a few years ago). Here's the thread that announced it last year:
http://www.belltreeforums.com/showt...-Eight-Years-The-Return-Of-The-Bell-Tree-Fair
It's basically a month and a half long event in which a new board is created for the event. A separate currency called "Tickets" were introduced for the event, and they could be collected by taking part in the events. The events ranged from cooking (posting the recipe), caption contests, writing, as well as a few luck-based ones such as "spin the wheel" - where you could take a chance to risk earning/losing tickets.
These tickets could be redeemed in the TBT shop for both digital and physical prizes. Some of the digital ones included the feathers -> Common ones (Red, Yellow, Green, & Blue) cost 5 tickets, Uncommon ones (Pink & Purple) were 10, and the elusive white feather was 15. There were also a variety of Steam games as prizes - around 15-30 tickets. As for physical prizes, there were stickers, pins, AC:NL Prima guides, and others that ranged from 10-80 or so tickets. (You could also buy raffle tickets to try to win a copy of AC:NL, homemade 3DS bags, Animal Crossing plushies, and eShop gift cards.
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Lots of different things! Depending on what you like, there's a variety of things to participate in as well as prizes. I'm sure that some things will probably be changed this year. The mods like to keep things interesting