Seasonality and calendar effects. Weak but persistent patterns driven by flows, tax dates, and holidays; September is the only month with a negative average return.
What to watch: Sell in May; September weakness; Santa rally; January effect; turn-of-month. It arrives annual, and the effect typically lasts weeks.
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.
Its neighbourhood
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Also in Market mechanics, derivatives, and calendar
- Zero-days-to-expiry (0DTE) options
- Monthly and quarterly options expiration
- Index rebalances and reconstitution
- Month- and quarter-end rebalancing
- 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