I'll use the longest day in the mid latitudes of the northern hemisphere to explain.

As the year progresses, the points on the horizon where the sun rises and sets move north (from mid-winter to mid-summer)) or south (from mid-summer to mid-winter), little by little, each day. You can pick that point by using two sighting marks far apart. It could be as simple as having two people, one to move a stick on the instructions of the other who eyeballs the sun and the stick from a fixed point. Some megalithic standing stones were probably for this purpose. On the longest day, sunrise and sunset are furthest north. The following day, the sunrise and sunset will move back south a small but discernible distance.