Co Loa is a commune belonging to Dong Anh, a suburban district of Hanoi city. Starting from Hanoi, we can reach the commune by going along Highway No 1, crossing Chuong Duong bridge and then Cau Duong bridge, and thereafter following Highway No 3 for three kilometres. Getting there, one can see the remains of three layers of concentric ramparts built by King An Duong Vuong, alias Thuc Phan, in the second century BC.

In the second century BC, Vietnam was a country named Au Lac ruled by King An Duong Vuong whose name prior to coronation was Thuc Phan. He had only one daughter, whose name was My Chau. The State's capital was located at Co Loa where the King built a fortress and ramparts. Yet, whatever was built in the day was subsequently demolished at night by evil spirits. The King held a religious ceremony and begged Heavens for assistance. Thereupon, a golden tortoise came and gave guidance to the King on how to do away with the malfeasance of evil spirits. As a result, the fortress and ramparts could be completed. In bidding farewell, the golden tortoise presented the King with one of its claws. The King turned the claw into the trigger of his crossbow, a single arrow of which could shoot thousands of enemy troops. The crossbow was thus called as the Magic Crossbow or Tortoise Crossbow.

By then, a rebel Chinese General, Trieu Da, established in southern China a kingdom called Nam Viet. He tried to conquer Au Lac State, but his military adventures ended in failure because of the magic Crossbow. Resorting to a different stratagem, Trieu Da sought peace and requested Thuc Phan to give consent to the marriage between Trong Thuy, Trieu Da's son and My Chau, Thuc Phan's daughter. Thuc Phan gave his agreement that Trong Thuy could, as son-in-law, stay in Au Lac Kingdom in keeping with the practice of uxorilocality with bride service.

Taking advantage of his wife's deep feelings of love and credulity, Trong Thuy stole the secrets of the magic crossbow. Thereafter, he sought permission from Thuc Phan to return home for visiting his parents. In departing, Trong Thuy said to his wife, 'In case war breaks out between our countries some time in the future, how can I find you?'. His thoughtless wife replied, 'I've got a coat made of goose feathers. I'll take them out and scatter them wherever I go.'

Thereafter, Trieu Da launched a new war of aggression against Au Lac. Being overconfident in his magic crossbow, Thuc Phan did not make adequate defence preparations. When enemy troops surrounded the Co Loa fortress, Thuc Phan started using his crossbow but found it useless. He had barely time to put his daughter on horseback and ride away at full gallop to the south. When he cam near the sea coast, the golden tortoise appeared. The King asked 'Oh, Golden Tortoise, why have I lost my Kingdom?'. Immediately the Tortoise replied, 'Your enemy is just behind you.' As the truth suddenly dawned on him, the King drew his sword and killed his daughter, and then followed the golden tortoise into the sea. The sea waves brought the corpse of My Chau to Co Loa, and when the villagers brought it ashore it became a big stone which is still kept and worshipped in a small temple near an old banyan tree. Trong Thuy also suffered from the loss of his wife. One day, he came to a well in front of the former palace of King An Duong Vuong. As he saw the face of My Chau on the surface of the water of the well, he plunged into the well and died. The blood which poured from My Chau's body into the sea - as a result of the cut from her father's sword - was swallowed by oysters and shells which subsequently produced pearls. Popular imagination even went so far to assert that if the pearls were washed in the water of the well where Trong Thuy died, they would become much brighter.

Every year, the 12 villages of Co Loa commune join hands in organising the festival, which usually begin on the 6^th of the first lunar month.

Early on the morning of the first day of the first lunar month, 12 notables from each village come to the house of the chief notable in order to prepare for the procession which starts soon thereafter. Opening the procession is the music band, to be followed by the chief notable and the 12 village notables, and then the villagers who carry various offerings to King An Duong Vuong.

A couple of stone horses, one red and one white with embroidered saddles, stand on both sides of the outside part of the gate of King An Duong Vuong temple. The road leading from the gate to the temple is lined with wooden stands for flags and other objects of worship. The palanquins of the 12 villages are placed in accordance with a pre-arranged order.

For the occasion, the organisers put in front of the temple a big altar with a glass box containing two gold ear rings and objects of worship. A smaller altar, containing the King's arms - sword, crossbow and bronze arrows - is set in front of the bigger altar. Beyond it is a space where a number of red-rimmed mats are spread and where the notables and the population will carry out the religious ceremony.

When the procession reaches the temple, a miniature royal court, containing an incense burner, a stone tablet, and the funeral oration, are put in front of the two altars.

The chief notable officiates at the religious ceremony in honour of the God King, which is in terms of rites quite similar to other traditional ceremonies, amidst the sound of music from the band. The notables are followed by the inhabitants. All beg the King to bestow peace and prosperity to the villages.

The ceremony lasts until 1 or 2 p.m., and is followed by a general procession, with the participation of all the 12 villages, in honour of the King.

In the front ranks of the procession are the flags, the Miniature Royal Court and the sacred weapons of the Temple. Then come the music band and the village notables clad in traditional court uniforms and holding the weapons supposedly wielded by the King - sword, crossbow and arrows.

Then come the notables and inhabitants of the Pagoda hamlet and of each of the 12 villages, with their own palanquins, flags and music bands. It is quite a long procession, which proceeds at a slow space, amidst the sound of music. Starting from the temple of King An Duong Vuong, the procession comes to the Trong Thuy well and then to the village gate. Thereupon, the contingent which carries the Miniature Royal Court and the royal tablets go back to the temple while the notables and population of the 12 villages would carry on the procession to their respective hamlets.

The religious ceremony and the procession end on the same day, that is, January 6, while the festival goes on until the 15^th of the same month, with various traditional games and activities:

In the night, there are fireworks, Ca Tru songs, folk theatrical plays (cheo) and conventional theatrical plays (tuong).

In the day, old men play chess and cards, old woman perform rites and present offerings at the pagoda, while young men and women and children have their own games: wrestling, tug-of-war, swing, rope climbing, martial arts, shooting from bows and crossbows, flag dancing, human chess (chess games in which human beings are used in place of traditional chess-men), cock-fighting, penny-pitching, rice-cooking contest and others. One ancient game which consisted in continuous drum-bearing either by pairs of drums or a whole groups of drums, is no longer played.

People from neighbouring communes used to come in their numbers to Co Loa to participate in the festival, regarding it as a national festival and spring merry-making.