Every 6 years, December 1 is a national holiday. It’s the day that Mexican federal power transfers to a new president. Out with the old, in with the new (unless you are Porfirio Diaz or Benito Juarez that is). It’s similar to the transfer of the flag from the graduating class to the junior class during graduation ceremonies.
Here you can see the transfer of power (represented by the flag) from Felipe Calderón Hinojosa to Enrique Peña Nieto in 2012.
I imagine this became a federal holiday to reduce protests bound to happen during such a momentous event. With no work, it’s a good chance that the peasants will be too drunk to do much in the way of organizing a revolution. But this remains my own opinion on it. I was unable to find any reason why this day is in any way special. It’s the same old tired story, no matter who is president.