Looks like this year it will be on November 9th. You can also find this info at Latacunga.gov.ec. Mama Negra Fiestas take place twice yearly. The larger and more colorful celebration is in November. The dates change from year to year- sometimes coinciding with Latacunga's founding and sometimes with the days surrounding Day of the Dead- so be sure to ask around.
Latacunga is not known throughout Ecuador just for chugchucaras but actually most for La Mama Negra Fiesta. It takes place twice a year and is a mixture of indigenous, Spanish, and African influences. Its origin is to celebrate the Virgen de la Merced (Virgin of Mercy) for stopping the eruption of Cotopaxi in 1742. A huge parade and street festival resplendent with street vendors, fireworks, dancing, and carnival rides ensues. The big one is September 23rd and 24th while a smaller one is on Latacunga's Independence day, November 11th.