John Mayer Throws Shade for Taylor Swift's Birthday

John Mayer celebrated Taylor Swift's birthday not by throwing a party, but by throwing MAJOR shade her way.

John tweeted a not-so-subtle diss at his ex Tuesday -- her 27th born day -- calling it "the lamest day of the year." He pulled the classic post-and-delete ... which only makes it more obvious it was a shot at Taylor.

Dear John clearly still has some bad blood with T Swift.

On a happier note -- happy birthday, Taylor? Belatedly.