Every year on summer solstice, the Sun rises at a point that is farther north than on any other day of the year. At the ruins of Stonehenge in England, this solstice sunrise appears on the horizon in direct alignment with the massive heel stone. This is the most outstanding feature of the monument and there is little doubt that the builders of Stonehenge used it to mark this special day as the beginning of each year. By counting the number of days between these annual alignments, they could determine the length of the year.

Knowing the basic calender via Stonehenge could help to determine planting and harvesting period

With this, the time for the equinoxes can be determined as well. This could serve as a practical, basic calendar to mark holidays and seasonal festivals and to ensure the timely planting and harvesting of crops. This is essential for the Druids as knowing the right time to plant and harvest would affect the crop yield which was vital to the Druids' survival since agriculture is one of their major sources of food.