When is the flea market on Animal Crossing

The Flea Market is a weekend event where villagers from your town allow others to come in and buy some of their items, although some of their furniture isn't for sale for various reasons. Many animals choose to sell items at a raised amount.
You can also sell your own belongings - any furniture out on your ground floor when villagers visit is up for sale. However, even if you run out of items to sell, the villager will take a few minutes to leave unless you leave yourself, then they'll ask if you need to go, and say they should leave. If you leave and re enter your house enough, every villager will come in once.
Items or creatures that a villager buys from your house usually will be displayed in their house the next day.
When you are buying from your neighbors you can either take the price they give you, or leave it. If one of them is buying from you however, they will haggle by saying the price is too high, and you have to lower it for them to buy it. Comically a cranky villager, when buying something from you, may hand you money that they claim is from their super secret giant bell hoard.
A villager will buy a max of three items from your house. However, if you go into a different room without leaving the house, then go back to where the villager is, it resets it so that the villager will buy another three items. This can be repeated as much as you wish. The maximum acceptable price they will pay is a little over twice the selling price to Tom Nook, which means for furniture, they will sell for half the buying cost. If you have bugs or insects in your house, the same rule applies, twice the selling price. So if you stock up on high priced fish/insects before or during a flea market day, you can make double amount of bells by placing them in your home and coaxing the villagers towards the creature you wish to sell.