>> There are no great universal midsummer festivals

May Day? Lammas/Lugnassa. There's a lot of summer festivals.

We have Easter in the spring & harvest festivals in Autumn.

Some of these may not be that important now, but they were more so when I was a kid.

Lugnassa festivals (which often involved pilgrimages to hilltop cairns & parties there) were still very, very common in 1960's Ireland.

Xmas became the big Xtian festival, so it's bound to dominate our minds these days.