SPX500 Day-of-Month Seasonality: ETF Days 1 and 16 Show Strong Historical Performance

A new SPX500 seasonality study reveals a significant edge on ETF-related trading days, specifically day 1 and day 16 of the month. Based on daily data from 1970 to 2026, these days show higher ROI, improved win rate, and stronger profit factor compared to average market behavior. This day-of-month effect suggests a structural seasonal pattern that traders can potentially exploit in systematic strategies.