It is the anniversary of the Battle of Puebla.
It isn’t a holiday in most other parts of Mexico besides the State of Puebla.
It wasn’t anything in the US until some bar owners promoted it in the LA basin where there were a lot of Mexican workers to have some celebration to sell more beer in the US.
Most people aren’t even sure where Puebla is, or really care.
More white people party on 5 May than Mexicans.
But hey, Viva Mexico!