Past summers with two big movies:
2018 — IW (if you count it)+I2 around 825M+740M adjusted
2015 — JW+AoU ~860+605 adjusted
2012 — TA+TDKR ~870+625 adjusted
2004 — Shrek 2+SM2 ~ 785+670M adjusted
1999 — TPM+Sixth Sense ~940+640M adjusted (first runs)
1996 — Ind Day+Twister ~770M+605M adjusted
However, if you look at just meteorological summer (June+July+august) rather than BO summer season, literally none of those qualify. For that you have to go all the way back to the massive twin smashes of 1994 — The Lion King, releasing wide on Jun 24, made a massive ~845M adjusted first run, followed a scant two weeks later by Forrest Gump on Jul 8th with a staggering ~895M adjusted
So we’re roughly running back the summer of 30 years ago but ~25% smaller