Why do you think ancient people paid close attention to the summer and winter solstices?

They believed that is was a marker for when crops should be harvested and some cultures believed that it was when the gods where coming.

The ancients noticed that as summer progressed, the sun stopped moving northward in the sky, then begin tracking southward again as summer turned to autumn. (During the winter solstice, the sun does the opposite, and begins moving northward as winter slowly turns to spring.