Equinox Holy Days

Why did the Neolithic people go to all the trouble of moving and erecting giant stones to build ancient monuments like Stonehenge? I think it was to to set the correct days for their religious rituals. The positions of the stones allowed them to track the movements of the sun and moon and so determine the  solstices and equinoxes, or what we now call the first days of the seasons. The vernal equinox, or first day of spring, is one such day.

If you think it strange to time religious holidays according to the sun and the moon, consider this. Easter is always the first Sunday following the first full moon after the vernal equinox. The vernal equinox this year is March 20, the next full moon occurs on March 27, and so Easter is the following Sunday, March 31.  Unlike Christmas, the day for Easter varies every year because it is essentially a lunar, not a solar, holy day.