From UO Excelsior Wiki - Ultima Online Free Shard
Grand Auction are held 1-4 times per year, roughly quarterly. They are large-scale auctions hosted by the staff and include high-end deco, weapons, armor, and other useful items.
Grand auction items are sold for gold and tokens only. They are announced a few days before the auction. They are held in Town Center in the room on the North side of town, directly behind the table that displays previous Grand Auctions' total sales. The auction items are displayed in the Auction Display Room, accessible through the green gate in the Auction room.
You may view the history of Grand Auctions on the public spreadsheet. It is maintained by +Nyx, and contains every auction she has hosted as well as any Sunday deco-dupe auctions hosted by +C in that time period. Grand Auction Data Sheet