In the Achaemenid era the Persian year was devided into to periods. One was the summer which was seven months long. The other was the winter, five months and five days long.
At that time celebrations were held at the beginning of each one of these periods. There were also symbols for each of these seasons. The lion was the symbol for the summer, and the cow was the symbol for the winter. The two are sculptured on the walls of the Persepolis engaged in fighting.
The fight is to represent the natural course of seasons. The triumph of the lion over the cow indicates the turn in nature toward the summer.
The ritual slaughtering of cows in Mithraism which can also be seen in Mithraic handicraft originates from the idea.
While separate figures of both the lion and the cow are present in the Persepolis buildings, both those parts that still remain in place and parts that are taken away to museums, the fighting scene is also recurrent.
Based on the psychology of Carl Gustav Jung, the fighting can be interpreted as a union of the two opposites, showing the ancient Persian belief that when two opposites are reconciled completion occurs. The two opposite sides of lion and cow, warmth and cold, summer and winter made up the complete life that existed in the mind of the ancient Iranian.
The figure of the cow and lion in war is very much reminiscent of the yang and yin symbol of East Asian mythologies.
Nowruz occupies a special place in the annual cycle perhaps because it is the time when the presence of both sides is most discernible, and perhaps also because the intermingling is the one that leads to an enlivening, a rebirth of nature, an ultimate proof that the union of the opposites has yielded to a full life that promises of verdure and prosperity.