What other people do, I believe 100%, is their own business. However, I believe the equionoxes and solstices should be celebrated on the day they occur, I read ALOT, my question is this: many books say to celebrate on the eve of the holiday, due in part to the Celtic belief that the day didn't start in the morning, instead starting when the sun sets, making the evening before the actual beginning of the day. As with Ostara, which is the time I celebrate the return of the Sun, the return of the Goddess from the Underworld, and the return of life in Nature, shouldn't that be celebrated with the rising sun?
Well, Full Moons are Esbats. People used to get together at Esbats because it was light(-ish) at night and because they could.
The Sabbats are the Solar festivals of Solstices and Equinoxes, and the fire festivals of Imbolc, Bealtaine, Lammas and Samhain.
For me personally, part of the celebration is celebrating the evolution of the Calendar, which marks these days accurately. Others find it more appropriate for example to celebrate Imbolc when they see the first snowdrop. Or Ostara when they see the first daffodil. Ye pays yer money and ye takes yer choice. I prefer to stick to the calendar and I don't observe the Esbats - partly because I don't do that kind of magical working, and partly because I don't care to spare the time.