Taylor Swift is a veteran of the stage, so it's befitting that she has commissioned a massive stage for her and Travis Kelce's wedding event at Madison Square Garden ... and it's being secretly built at a Pennsylvania production facility ... sources tell TMZ.

Our sources say the stage is being built at a campus called Rock Lititz in Lititz, Pennsylvania. It's a massive facility where bands often rehearse before going on tour. It's also where stages for various tours are constructed.

We're told a "massive stage" is being constructed in a warehouse "under lock and key with security guards patrolling the area." Our sources say the stage is not affiliated with any present or upcoming music tour.

Taylor has history with Rock Lititz ... that's where Taylor's massive video columns were constructed for her 2018 Reputation Stadium Tour. Clair Global, a subsidiary of Rock Lititz, also worked on the sound for her Eras Tour.

We're told a band will obviously be present, but some of Travis and Taylor's guests will be invited to hop on stage and take the mic. It's a fringe benefit of having REALLY talented friends!

It is increasingly looking like MSG will be the venue for the wedding celebration, but not the wedding itself. Several guests tell TMZ ... they believe the wedding will take place before the July 3rd MSG event, and the guest list is much smaller than the list of over 1,000 MSG-invitees.