La Nueva Guatemala de la Asunción, is the complete name of Guatemala city, a name give by Carlos III with a royal license on May 23rd 1776, thus the reason why today August 15th, the day celebrated as the Assumption of Mary, is the official day of Guatemala city.
La Nueva Guatemala de la Asunción is the fourth and current capital of Guatemala. It was established the 2nd of February 1776 on the area now known as Zone 6 on the Valle de las Vacas or Valle de la Ermita, where the Church of la Parroquia stood. The church had an image of the Virgen de la Asunción who became the mother patron of the new city. In 1804 the image found a new home in the Church built for her in the north side of the new city. This continues to be her home and the place were the Fair of Jocotenango in honor of the Virgen de la Asunción takes place every 13th, 14th and 15th of August.
The seal of arms of Guatemala city, whose name comes from the Nahuatl Goatemalan, which means land of many trees, is the same seal given by Carlos I in 1532 for the establishment of the first capital in Iximche, later to the Valley of Almolonga, Ciudad Vieja, then moved to today's "Antigua Guatemala" and finally moved again to the new valley were it is today.
The Seal has Santiago the Apostle on top and three volcanoes in the bottom with three trees in front.
References: Tu Muni webpage