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1.
The holiest of holy days, Uno de Juno was originally a holiday created for non-mexican Spanish speakers. Sick of people assuming since they spoke Spanish that they wanted to celebrate the Mexican holiday of Cinco de Mayo, some Puerto Ricans and their white compatriots in New Orleans adopted Uno de Juno as their own day of holy celebration.

Intentionally misspelled, the first and only rule of Uno de Juno is that it is not celebrated on June 1st and the celebration usually consists of heavy drinking, drug taking and party-goers wearing poorly designed t-shirts.
Everyone got blackout drunk and Sam pissed himself, another great Uno de Juno was had by all.
by Dooder11 June 14, 2013
 
2.
A holiday in which you goto Chipotle on June 1st, play uno, and eat guacamole & chips.
Person 1: Dude, are you ready for Uno De Juno tomorrow?
Person 2: Yea man!! I made my shirt yesterday and everything!
by Payback451 June 01, 2006
 
3.
Celebrated annually on the first of June, this is the largest and most important festival in Mexico's busy festival calendar. Though little known by non-natives, it is especially popular among New York immigrants from Mexico's former state of Alta California.

Him: "I'm going to Scott and Trac's Uno de Juno party this year. Are you?"
Her: "Of course! Isn't everyone?"
by Informed Citizen May 17, 2007