“The only downside to Cinco de Mayo is Seis de Hangover” (or simply “Happy Seis de Hangover”) is a jocular saying about the day after May 5th (Cinco de Mayo) that has been printed on many images. “Wishes you a happy Seis de Hangover!” was posted on Twitter  by John Bartle on May 6, 2009. “The only downside to Cinco de Mayo is Seis de Hangover… http://some.ly/IPhfyB  vía @someecards” was posted on Twitter by Kurthe LSA + on May 5, 2012.
 
“Seis de Mayo sucks ass” is another saying about May 6th.
   
     
Wikipedia: Cinco de Mayo
Cinco de Mayo (pronounced [ˈsiŋko ðe ˈmaʝo] in Latin America, Spanish for “Fifth of May”) is an annual celebration held on May 5. The date is observed to commemorate the Mexican Army’s victory over the French Empire at the Battle of Puebla, on May 5, 1862, under the leadership of General Ignacio Zaragoza. The victory of the smaller Mexican force against a larger French force was a boost to morale for the Mexicans. A year after the battle, a larger French force defeated Zaragoza at the Second Battle of Puebla, and Mexico City soon fell to the invaders.
 
In the United States, Cinco de Mayo has taken on a significance beyond that in Mexico. More popularly celebrated in the United States than Mexico, the date has become associated with the celebration of Mexican-American culture.
         
Twitter 
John Bartle
@johnbartle
Wishes you a happy Seis de Hangover!
8:39 AM - 6 May 2009
   
Twitter
katie friel
@katiefriel
Seis de hangover.
10:01 AM - 6 May 2009
   
Twitter
Billy Dee’s Tweet Shack
@bfd
says “Good morning - it’s the Seis de Hangover.”
11:40 AM - 6 May 2009
   
Twitter
neilhenderson
@neilhenderson
Replying to @krislayman
@krislayman Star wars day, synco de mayonaise, seis de hangover. Too many holidays! I can’t keep up!
1:33 PM - 6 May 2009
   
Twitter
Lauren bailey
@Callheraplum
Cinco de drunk…. Seis de hangover 😊 http://twitpic.com/1lefyz
10:00 PM - 5 May 2010
   
Twitter
edi edi
@MrEddieBentley
Cinco de drinko is O V E R. Seis de hangover H E-L L O
3:34 AM - 6 May 2010
   
Twitter
andrea 🌙 rose
@X0X0angiee
Happy Seis de Hangover.
7:11 AM - 6 May 2010
   
Twitter
ThingsNatSays
@ThingsNatSay
LOL RT @jesus   Enjoying a lovely seis de hangover.
2:09 PM - 6 May 2010
   
Twitter
Adrienne
@adrienneMTK
Remember, kids: Cinco de Mayo becomes Seis de Hangover in the span of three tequilas.
11:36 AM - 3 May 2011
     
Twitter
Kurthe LSA +
@kurthe_ndk
The only downside to Cinco de Mayo is Seis de Hangover… http://some.ly/IPhfyB  vía @someecards
2:36 AM - 5 May 2012
   
Twitter
La Puerta
@LaPuertaSD
The only downfall of Cinco de Mayo is seis de hangover. Swing by for $1 Mexi-mimosa brunch with best carne asada & egg burrito in town.
2:07 PM - 6 May 2012
   
Facebook
Piola USA
May 3, 2013 ·
The only downside to Cinco De Mayo is Seis De Hangover!
Happy Cinco De Mayo! Come to Piola and Celebrate with us!
 
Twitter 
Taylor
@tayhidesfromHR
the only downside to Cinco de Mayo is Seis de Hangover #truth
10:30 PM - 5 May 2013
 
San Antonio (TX) Current
Taco Cabana to Cure Your Cinco de Mayo Hangover with Free Tacos
Posted By Alejandra Lopez Gonzalez on Mon, May 2, 2016 at 2:22 pm
(...)
Lookout for Seis de Hangover coupons on May 5th on Taco Cabana’s Facebook page, or follow them @TacoCabana on both Twitter and Instagram. But just in case you’re turning up too hard on Thursday to scroll through and find the coupon, we’ve also included it here. 
 
4 May 2016, La Prensa (San Antonio, TX), “Five places to celebrate Cinco de Mayo in S.A.” by Lucy Almanza, pg. 6:
Taco Cabana will celebrate its annual “Seis de Hangover” on Friday, May 6 with a free taco giveaway during breakfast hours. Guests can access the coupon on Taco Cabana’s social media channels on May 5 for their free breakfast taco (Facebook: http://www.facebook.com/tacocabana; Twitter and Instagram: @TacoCabana).
       
Twitter
A
@amberlynn4L
cinco de drinko was fun but seis de hangover is in full effect. 🙄
12:20 PM - 6 May 2016
   
Twitter
lεรlﻨε
@_leslie28_
May 6…seis de hangover😴😴
9:37 PM - 6 May 2018
 
El Tecolote
The real reason why gringos celebrate Cinco de Mayo
May 17, 2018 Arturo Mendez
Cinco de Mayo is a very popular celebration in the United States, followed of course by another tradition “Seis de Hangover.” But contrary to popular belief, Cinco de Mayo is not Mexico’s Independence Day, nor even a date when Mexicans celebrate or party as a nation. On May 5, we remember the famous battle of Puebla (my hometown), when Mexico defeated the French army, which at the time was considered the strongest in the world.
 
Twitter
💋SG 🃏
@sko12
Replying to @Joyannah73 @jvbt82hhs
Followed by Seis de Hangover
6:57 PM - 30 Apr 2019