I was just rereading my kid's book about the Celtic holidays and how they are celebrated today (pretty much the title right there

) and one of the things the authors point out is that the Celts were agricultural people. They celebrated the milestones of the yearly cycle when they happened.
You don't celebrate the harvest until you actually have the harvest. You don't celebrate the lambing and such early signs of Spring if they haven't happened yet. You don't celebrate the start of Summer when the crops have been planted until you've done so.
Unlike our calendars which are rigid on what days are when, what holidays are when rather...they were flexible about it. They had to be.

I have also noticed that there is a change around Samhain, give or take a week or 2, a different feel to the air, that just doesn't exist with the Equinoxes. Same for the Spring end of the year. It's probably different at a different latitude, but for mine, those dates do not particularly correlate with a seasonal change from what I've observed.