Katy Perry & John Mayer -- Valentine's Day ... A Good Day to Die Hard

John Mayer might just be the luckiest guy on the planet -- because he spent his Valentine's Day alone in a dark room with Katy Perry ... and more importantly, Katy Perry's boobs.

J & K celebrated the holiday of love last night by hitting up the historic Vista Theatre in East Hollywood -- and the photographer who shot the pics tells us, he didn't see a soul enter or leave the theater the entire time they were in there ... roughly two hours.

The theater wouldn't comment on whether John and Katy rented the place out for their own personal use -- but that's the only logical conclusion. Pretty baller.

"A Good Day to Die Hard" is the only movie currently playing at Vista ... but it's unclear if that's what the couple actually watched ... because Katy was spotted leaving the place holding a DVD.