Events need observers.

Do we create solstices and equinoxes by observing them, such that they are man-made anyway?

Ouch. I bet you were probably just being coy, but you should understand that Schrödinger's wave equation (and the alternate but mathematically equivalent explanations of Quantum Mechanics by Feynman and Heisenberg) do not say that events occur only if there are observers - quite the opposite! QM actually requires the existence of all possible outcomes (this is called the "universal wavefunction"). But for a specific observer, the act of observation 'collapses' this wave of possible outcomes for that observer into one specific solution/event.

So, yeah, equinoxes and solstices still occur if no one is around to observe them - the universe, according to QM, acts deterministically (like a clockwork) until observed. Randomness (at the quantum level) is only introduced by observation.

(Yeah, I know, it makes my head hurt too. But QM is literally the only mathematically valid explanation that accounts for (and has predicted) things like semiconductors, lasers, etc. And believe me, there are a gazillion physicists that have been trying to find some working alternative to QM for decades - it makes a lot of their heads hurt as well, and physicists hate headaches.)

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It's a tough time to be a pagan :(

Fwiw, I have observed that neopagans (in this part of the world) are likely to find the weirdness of QM less threatening to their personal traditions and beliefs than do members of the Abrahamic religions (Judaism, Christianity and Islam). Perhaps not coincidentally, there's many neopagans in the academic and scientific community here in Eastern MA.