Carnival is a festive season which occurs immediately before Lent and is traditionally held in Roman Catholic societies. http://en.wikipedia.org/wiki/Carnival#Mexico Mardi Gras in New Orleans is an example of this event in the United States.
Carnaval en Mérida was a week-long celebration that involved the whole city. I thought it would be fun to go see one evening, but many said they had never gone, or that it was crude, so we didn't even try to participate. The center of the city was blocked off during the nightly parades and the weekend mid-day activities, which meant that participants at the temple had to take a round-about route. Several workers didn't come and youth baptisms were noticeably absent. As luck would have it, that was the week Salt Lake asked us to keep attendance at the temple, so our report was very low. Saturday made up for it with 6 busloads from Campeche and Cancun in the morning, then Motul at noon.
We visited Quintero ward (northeast of the city), then ate dinner with the Pérez family; by the time we came home on Sunday about 3:00 P.M. the activities were over. All we saw was lots of people, many with costumes or masks, and the streets were still closed. We had to go to 79th Street, about 6 streets north, before we could come west to 72nd Street where we turn into our apartment.

Del 18 al 25 de febrero, la algarabía y calidez de los yucatecos se dejan sentir en "Un Carnaval de Pelicula", llevando alegría y fiesta a la gente que se encuentre en Yucatán. Es el carnaval de Mérida, unos de los cinco más importantes en la República Mexicana y de los 12 principales de América Latina. http://yucatantoday.com/es/topics/carnaval-de-merida-2009-un-ca
Newspaper pictures from Diario de Yucatán