In the mid-20th-century U.S., the celebration of Cinco de Mayo became among Mexican immigrants a way of encouraging pride in their Mexican heritage. Today it is celebrated all over the world with the promotion of Mexican beverages (i.e. tequila), cuisine (tacos), and assorted festivities (pinata parties).
IN OTHER WORDS:
It's the day Americans celebrated Mexico and all it has brought into the U.S.
However it got past the wall...lol