Circles of big stones like Stonehenge were rebuilt so that the sun's position with respect to the stones would indicate the day of longest sunlight and the day of shortest sunlight. Between these days there was an optimum time to harvest the crops before fall, when plants dried up and leaves fell from the trees. The winter solstice, when the days began to get longer was cause for celebration. In the next season, there was an optimum time to plant seeds so they could spring up from the ground as new growth. So farming gave rise to the concept of a year. Certain changes of the year were celebrated, such as Easter, named for the Goddess of the Dawn, which occurred in the east (after lent); May Day celebrating the revival of life; Lammas around July, when the wheat crop was ready for harvesting; and on October 31 the Celtic eve of Samhain, when the spirits of the dead came back to visit homes and demand food or else cast an evil spell on the refusing homes; and at which masked and costumed inhabitants representing the souls of the dead paraded to the outskirts of the settlements to lead the ghosts away from their homes; and at which animals and humans, who might be deemed to be possessed by spirits, were sacrificed or killed perhaps as examples, in huge bonfires [bonefires] as those assembled looked out for spirits and evil beings. There was an agricultural revolution from the two-field to the three-field system, in which there were three large fields for the heavy and fertile land. Each field was divided into long and narrow strips. Each strip represented a day's work with the plough. One field had wheat, or perhaps rye, another had barley, oats, beans, or peas, and the third was fallow. These were rotated yearly. There was a newly invented plough that was heavy and made of wood and later had an attached iron blade. The plough had a mould-board which caught the soil stirred by the plough blade and threw it into a ridge alongside the furrow dug by the plough blade. This plough was too heavy for two oxen and was pulled by a team of about eight to ten oxen. Each ox was owned by a different man as was the plough, because no one peasant could afford the complete set. Each freeman was allotted certain strips in each field to bear crops. His strips were far from each other, which insured some very fertile and some only fair soil, and some land near his village dwelling and some far away. These strips he cultivated, sowed with seed, and harvested for himself and his family. After the harvest, they reverted to common ownership for grazing by pigs, sheep, and geese. As soon as haymaking was over, the meadows became common grazingland for horses, cows, and oxen. Not just any inhabitant, but usually only those who owned a piece of land in the parish were entitled to graze their animals on the common land, and each owner had this right of pasture for a definite number of animals. The faster horse replaced the ox as the primary work animal.