Backstreet Boys Post Video of Howie Dorough Taking Shots For Cinco De Mayo

The Backstreet Boys are giving people ideas on how to spend Cinco de Mayo ... showing fans how the guys celebrate -- with a vid of Howie Dorough slamming shots.

The band's official account posted the video Sunday morning ... and, it follows HD's drinking odyssey as he fires down tequila shot after tequila shot at Howie's Guac and Shots.

And, man ... Howie really knows how to hold his liquor -- downing at least eight shots in the sweltering Mexico heat while apparently MCing one of the events.

The band's account captioned the clip, "Happy Cinco de Mayo…. this is HOWIE celebrate."

Apparently, the event in the video took place a couple weeks ago at "Backstreet's Back at the Beach" ... a festival organized by the group in Cancún -- but, as far as we can tell, this is the first time they've posted a vid counting off drinks. Seems they saved it for a special occasion.

The event celebrated 30 years of the Backstreet Boys and featured curated events for fans including a series of live performances ... one rocking good time for sure.

BTW ... the Backstreet Boys have a ton of time to celebrate Cinco de Mayo weekend 'cause they were slated to appear at the Lovers & Friends Festival in Las Vegas before it was canceled.