OMG -- Taylor Swift Just Dressed As Olaf Onstage And Sang 'Let It Go' With Idina Menzel

Yeah, it happened.

Tampa, Florida -- "It's Halloween," Taylor Swift said, sparkle in her eye. "So you never know what might happen."

It was the type of coy attitude that we're used to from our girl Tay Tay when she teases the upcoming special guests on her 1989 Tour, but this time, she REALLY followed through.

So, did Taylor get in the holiday spirit? You bet.

Fast forward to "Style," where Taylor usually struts down the catwalk in a shiny diamond bodysuit -- but this time it was different. This time, she emerged onstage as Olaf. Yes, Olaf -- the big, lovable snowman from "Frozen." Her dancers were reindeers. Her fans, well, they were stunned.

"I don't know if you noticed but we have a 'Frozen' theme going on here," Swift said. "If only Idina Menzel were here."

And guess, what? That sly fox brought out Idina Menzel! THE Idina Menzel of "Let It Go" fame.

Menzel came up through the trap door wearing a baby blue gown, just like Elsa does in "Frozen" when she's busting out in song in her ice castle. Soon enough, her and Taylor were trading verses, and Swifties were singing at the top of their lungs.

"The cold never bothered me anyway," the sang sassily in unison. Idina and Taylor shed their costumes with angst and theatric passion and made their way down the catwalk.

Not only did Taylor dress up as Olaf, but her band dressed up too. Her guitarist wore platform shoes while the drummer donned a white wig. The dancers were having even more fun than usual, wearing masks for "Shake It Off" at the end.

So, yeah, best Halloween ever.