1 : literally, the fifth of May
2a : a holiday celebrated by millions of Mexican nationals in America with notable festivity and drunkenness to commemorate their heritage and pride in the greatness of a country they couldn't wait to leave
2b : a holiday celebrated in Mexico by a small number of Mexicans with overwrought sadness and drunkenness to lament their expulsion from America by the Border Patrol <see also, el día de los muertos>
We wildly waved the la Bandera de México at the East LA Cinco de Mayo festival to protest those nasty gringos who won't defund the Border Patrol so they can afford to increase our welfare checks and medical care.
"Happy Cinco de Mayo! Now go kill some Gringo Bastardos!" Jose Angel Gutierrez