There are a few dishes that only come around this time of year, the most famous being CHILES EN NOGADA.
Chile en Nogada means chilie in a walnut sauce and that is precisely what it is, a poblano pepper bathed in a sauce made from walnuts then sprinkled with pomegranate seeds. The idea is for it to have the colors of the flag. This dish is said to be made by a group of nuns in Puebla Mexico for Agustin de Iturbide, Emperor of Mexico from 1822 to 1823. After Mexico won its independence from Spain in 1821 de Iturbide signed the Treaty of Cordoba in Varacruz and then made his way back to Mexico City. He stopped in Puebla, about an hr south of the capital, and chile en nogada was born.
It can only be made in the months of August and September because it is when the pomegranates are in season and the walnuts are prime for making the sauce. Since the dish represents the colors of the flag and is made during the time of Mexico's Independence it is considered a very patriotic dish.
You can find this dish all over the city during this time. Simply drive around and look for the signs. Also, it is well worth the traffic to visit The Plaza Mayor or El Zocalo to see the lights and decorations.