Something extremely rare is about to happen to the S&P 500.

The benchmark index, as tracked by the SPDR S&P 500 ETF Trust (NYSE:SPY) — is on track to mark its ninth straight winning week on Friday, a streak matched only 10 times since 1945.

The run has lifted the index 18.76% off its late-March lows, erasing the selloff that followed the closure of the Strait of Hormuz in late February.

Buying at record highs after nine green weeks feels like chasing. The historical data says waiting is the costlier instinct.

S&P 500 Logs 9 Green Weeks: The Last 10 Times, Stocks Kept Climbing

In the 10 completed episodes, the index was higher one month later 90% of the time, with an average gain of 1.68%, TradingView data shows.

Three months out, the win rate slipped to 60% and the average gain to 3.01%.

Six months out, it was 70% and 6.04%. A full year later, the index was higher in eight of ten cases, an 80% win rate, with an average return of 10.21%. ...