Opening and closing auctions (leilão)

Markets do not just flip on and off. Each trading day opens and closes with an auction (in Brazil, leilão) — a special phase that gathers orders and finds a single fair starting or ending price.

Why an auction instead of continuous trading

At the open, orders have piled up overnight reacting to news. If trading simply started, the first few orders could execute at wild, unrepresentative prices. The auction solves this by:

1. Collecting all buy and sell orders during a pre-open window without executing any of them.
2. Calculating the single price that would match the largest possible volume.
3. Executing every eligible order at that one theoretical price when the auction closes.

The result is an orderly opening price that reflects everyone's interest at once.

Opening, closing and special auctions

• The opening auction sets the day's first price.
• The closing auction sets the official closing price — important because many funds and indices are valued on it.
• Volatility auctions can be triggered intraday if a stock's price jumps too far too fast, pausing trading briefly to let the market reprice calmly.

What this means for you

Orders placed during an auction window do not execute instantly — they wait for the auction to resolve. If you see your order sitting unfilled right at the open or close, it may simply be in the auction. This is normal exchange behaviour, not an error.