Mexican Flag Description
Centered in the white band is an eagle perched on a cactus with a snake in its beak, with bands of equal size in green on the left and red on the right.
The Mexican flag has varied over the course of the country’s history. When Miguel Hidalgo y Costilla declared independence in 1810, he carried the standard of the Virgin of Guadalupe. This is considered to be the first Mexican flag.
In 1813, the revolutionaries designed a new flag. They used white silk with a border of blue and white squares. In the center, an eagle perched on a cactus. This image was encircled by a Latin motto, Oculis et unguibus asqué victrix (victorious with both eyes and talons), and was topped with the Spanish crown.
The national flag was created in 1821 and adopted by the “Army of Three Guarantees,” which took its name from the guarantees of religion, independence and union promised by the Plan of Iguala. The three colors of the Mexican flag were adopted at that time: green for independence, white for religion, and red for union.
During the short-lived first Mexican empire, the stripes changed position. The eagle was placed alone on the white band, facing forwards with its wings spread. Its head was shown in profile bearing a crown.
Later, in 1823, Congress decreed that the emblem on the flag should conform to the indigenous myth: an eagle in profile perched on a cactus, eating a snake. 
The crown was eliminated.
The national seal was first added during Maximilian’s rule, when a flag bearing the eagle under the crown of the French empire was adopted. Porfirio Diaz subsequently ordered that the eagle on the seal be shown facing forwards with its wings extended. Later, under Venustiano Carranza, it was modified again: the eagle is now shown in left profile and reflects the Mexica legend of the founding of Tenochtitlan . A green, white and red ribbon representing the patriotic nation was added, shown tying together branches of oak and laurel.
Flag Day has been celebrated in every year since 1937 in a ceremony at the monument to General Vicente Guerrero, the first military leader to swear allegiance to the flag in Acatempan on March 12, 1821.