What does the following mean in English? el Dia de los Grandes
Literally, as you've probably already found, it is 'the Day of the Great.' Does that make sense to you?
If it is an expression then I'm not familiar with that one. And a web search didn't turn up anything easily. Where did you see/hear it? In what context?
Diá de los muertos
1 Nov. day that the spirits of dead children (angelitos) return to visit the living.
2 Nov. day that the spirits of dead adults (los Grandes) return to visit the living.
I found this article through a Google search. It appears that El Día de Los Grandes is a part of the larger Los Días de Los Muertos observations.
Read [This Article] about Los Días de Los Muertos and pay attention to the last sentence in the 2nd paragraph for a reference to El Día de Los Grandes.
I hope this is the information you wanted.
: http://ode.k12.ar.us/renteriap/PDFs/Projects/dia de los muertos/deadDiadelos muertospacket.pdf