When: Every 3 SkyBlock days (every real-life hour).
The Dark Auction is a place where you can bid on exclusive items. It is recommended that you take a lot of Coins though, as the items sold here go for quite a lot.
For a very short amount of time, an NPC called Sirius spawns outside of a house near Tia the Fairy. Speak to him to enter the Dark Auction. The Dark Auction starts at the 55th minute of every real-life hour (e.g. 9:55). It is recommended to have as many coins in your purse as possible to have a higher chance at getting in, since only the people with the highest money on them will be let in. There Are 4 Total Bouts of Bidding, each for a different item, the last item always being a flower minion. You can find a timer on when the event happens here.