Zero-days-to-expiry (0DTE) options. Half of index option volume now expires the same day; intraday reversals and late-day moves are increasingly mechanical.
What to watch: 0DTE share of SPX volume. It arrives daily, and the effect typically lasts intraday.
It sits in tier 3 — Flows and mechanics: Moves prices without changing anything fundamental, on horizons from hours to weeks. Who is forced to buy or sell, and when. Ignore this tier and market moves will regularly look inexplicable.
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Also in Market mechanics, derivatives, and calendar
- Monthly and quarterly options expiration
- Index rebalances and reconstitution
- Month- and quarter-end rebalancing
- Seasonality and calendar effects
- Tax-loss harvesting and tax dates
- Holiday and low-liquidity sessions
- Market microstructure and liquidity
- Circuit breakers and trading halts
- Settlement, clearing, and margin requirements
- Fed communication blackout
- Corporate buyback blackout windows
- Investor conferences and analyst days
- Jackson Hole and set-piece speeches
- 24-hour and tokenised trading
- Tax refunds and payroll-day flows
- Earnings season timing and pre-announcements