September 14 is over the MexicanIndependence Day weekend, which is always a hot date on the boxing calendar along with the first week in May for Cinco de Mayo.
The American independence movement had inspired Mexican leaders of that era and since Mexico, in fact, also was a territory composed of states, the name stuck and became official in 1824.
He says he will form an alternative government, and he's chosen to announce that government on the 16th of September in the same place and on the same day the Mexican military celebrates Mexico's independence day.