Bitcoin heads into the September 18 quarterly expiry with the worst seasonal record of the year, a warning sign for traders holding through mid-month.

• Quad witching on September 18 is when a pile of futures and options contracts expire at once, and it often marks a turning point in markets.
• Since Bitcoin futures started trading in 2018, the week after September's expiry has been down every year but one, losing about 2.5% on average.
• September is the odd one out — the weeks after the March, June, and December expiries all averaged gains of roughly 1%.
• The run-up into the expiry usually looks fine, with a small gain, which is what makes the drop afterward catch people off guard.

Outlook: Expect a soft stretch for Bitcoin in the week following September 18 if the pattern holds.