The Sun of May is also connected to Sol Invictus ("The Unconquered Sun"), a Roman god identified with the Sun (the main solar deity in the ancient Roman religion). This links it to the god Mitra Sol Invictus, a solar god whose worship the Roman emperor Aurelian made official throughout the Roman Empire.
The specification "of May" is a reference to the May Revolution which took place in the week from 18 to 25 May 1810, which marked the beginning of the independence from the Spanish Empire for the countries that were then part of the Viceroyalty of the Río de la Plata. A legend claims that as the new government was proclaimed, the sun broke through the clouds, which was seen as a good omen.
In the flag of Argentina, the Sun of May is the radiant golden yellow sun bearing the human face and thirty-two rays that alternate between sixteen straight and sixteen wavy.
In the flag of Uruguay, the Sun of May is the golden yellow sun bearing the human face and sixteen triangular rays that alternate between eight straight and eight wavy.
In form, it is similar to--and may be partially derived from--the sun in splendour, which is common in European heraldry. This, too, is usually depicted with a face, and with alternating straight and wavy rays (representing light and heat respectively), though it normally has only sixteen rays.
A 1978 law describing the official ceremonial flag of Argentina specifies that the sun must be golden yellow in color (amarillo oro), have an inner diameter of 10 cm, and an outer diameter of 25 cm (the diameter of the sun equals the height of the white stripe, and the sun's face is of its height), must feature 32 rays (16 undulated and 16 straight in alternation), and must be embroidered in the official ceremonial flag.
Es un sol figurado con rostro humano, de color oro amarillo con treinta y dos rayos: 16 flamígeros apuntando o "girando" en sentido horario, y 16 rectos colocados alternativamente, según diseño de la primera moneda argentina.
Media related to Sun of May at Wikimedia Commons