Sessions and auctions: the shape of a trading day

After this lesson you'll know the shape of a trading day, why the open behaves unlike any other hour, and what the closing auction is.

The day has a shape

Take the US market, where most of this Academy's examples live (European venues run the same shape on local hours; for a learner in Amsterdam, US regular hours are 15:30–22:00 CET). The timeline: pre-market trading from early morning, the opening auction at 9:30, regular hours until 16:00, the closing auction at 16:00, then after-hours until 20:00.

Pre-market. Only some participants show up, so the books are thin and spreads are wide. Overnight news gets its first pricing here, and it isn't a reliable one: prices move a long way on very few shares.

The opening auction. At 9:30 the exchange runs a literal single-price auction: all the orders accumulated overnight are matched at the one price that clears the most volume, and that becomes the official open. The half hour that follows is the most violent stretch of the day, because a whole night of information from around the world is being turned into positions at once. Beginners are drawn to the open because it moves. Plenty of professionals trade nothing before 10:00, and the open is a skill worth coming back to once Schools II and III have done their work.

Regular hours. Liquidity is at its deepest here and spreads at their tightest, with a slow stretch around lunch.

The closing auction. The biggest liquidity event of the day. Index funds (course 7) need to match their benchmark, which is computed on official closing prices, so their orders pile into the close. On rebalancing days a large share of the day's entire volume trades in that one print. The price it produces is the close your charts and every fund report are quoting.

After-hours. Earnings land here, usually within minutes of the close. The books are thin and the gaps are large, and the first traded price after a news release is best treated as a draft, with the real repricing happening at the next morning's open. School II, course 6 explains what this session does to your stops, which is the part that will affect you most.

Check yourself

1. Why is the first half hour the wildest part of the day? (A night's worth of information is being repriced at once, through the auction and just after it.)
2. Why do index funds trade at the close? (They track benchmarks computed on official closing prices; trading the close removes the difference.)
3. Earnings drop at 16:05. Where does price discovery happen, and what is it like? (After-hours: thin, wide spreads, gappy. The first price is a draft; the real repricing is the next open.)

The idea this lesson installs

Liquidity keeps a schedule; know where in it you're standing.

Next: Course 7 — "Indices and ETFs: what the yardstick is made of."