Just like any auction in the ‘real world’, auctions inside of Sunflower-Land work just about the same.
Generally auctions happen ‘once’ per currency per Chapter.
But the number of auctions for each item depends on the effect of the item in question; and the number of that item available.
When an auction is ‘live’ (able to be bid on), you will see a pop-up on the bottom of your screen saying as such.
At the auction house. Which is located just to the right as you land inside the Plaza. (on your game map, inside of the globe icon)
To participate, talk to the Auctioneer when one of the auctions is live.

There is generally only 1 type of Auction; the “Blind Auction".
There are 3 tokens that are used in the auctions; Flower, Gems, and Chapter Tickets.
Each of which happens (generally) at a different time period from one another. But the team has used many different scheduling methods over the chapters. And at the time of my writing this, there is no clear defined pattern as of yet.
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