'The Masked Singer' Has Longest Day of Production Ahead of Final Round

"The Masked Singer" is making history even before the big season finale ... with the cast and crew pulling off a nearly 24-hour day of production.

Our 'MS' sources say the final day of filming broke the show's record for the longest production day ... starting with a 6 AM call time, and wrapping up around 2:30 AM!

We're told, the crew pulled out all the OT stops to go out with a bang this season -- like adding all the lighting, extra stage prep and pyrotechnics into an already packed schedule.

The marathon production is gearing up for the last round of competition where 3 final costumed celebs -- Prince, Firefly and Ringmaster -- will square off for the ultimate title. And, we've learned, the competition is more fierce than ever.

On top of already having a time-consuming day, we're told the mystery performers wanted extended rehearsal time with vocal coaches and choreography teams ... ahead of the final, winner-take-all round.

Of course, ya also gotta add in some production bumps along the way. We're told the crew had to add some last-minute safety precautions to the costumes.