Before daily expiries, SPY's core weekly expirations were Monday, Wednesday, and Friday (Friday being the long-standing standard). Even now that SPY expires every weekday, the traditional weekly expirations — especially Friday — still tend to carry the heaviest positioning, which matters for how the tape behaves.
The evolution to daily
SPY expirations expanded over time: from monthly, to weekly Fridays, to added Monday and Wednesday weeklies, and finally to every weekday. So today there's a 0DTE each session, but the days that were weeklies longest (and the monthly third-Friday) often still concentrate the most open interest — a legacy of where positioning has traditionally clustered.
Why the big expirations still matter
Heavier positioning means stronger dealer-hedging effects: the days with the most open interest tend to have more pronounced pinning and expiration dynamics, and the monthly and quarterly (quad-witching) expirations are the biggest of all. So while you can trade 0DTE any day, knowing which expiration carries the weight helps you read the day's structure — a light-positioning day trades differently from a heavy Friday.
Every weekday has a 0DTE now, but they're not all equal — the traditional weeklies still tend to carry the heaviest positioning, and heavy positioning bends the tape.
The quick takeaway
SPY's weekly expirations (historically Mon/Wed/Fri, with Friday the heavyweight) still tend to concentrate positioning even in a daily-expiry world. Knowing which expiration is “big” helps you anticipate pinning and expiration effects. NoVo maps the live positioning and walls each day, so you see where the weight actually sits regardless of the calendar.