12/26 -- Sources say the proposer decided not to do a public engagement.

Someone's gonna pop the question during the live segment of the 'Survivor' finale later this week -- oh, and by the way ... this story contains spoilers. Sooooo, yeah.

Production sources tell TMZ ... 'Survivor' production staffers working on this year's 'David vs. Goliath' season received an email over the weekend alerting them to some extremely last minute changes for Wednesday's finale episode. The message ... we're gonna have a live proposal on our hands, folks.

We're told the staff was instructed to make some last-minute accommodations for the 3-hour finale -- part of which is live and the other being pre-taped from Fiji -- in anticipation of someone getting on bended knee in front of the live studio audience.

Unclear who's proposing, but we're told 'Survivor' staff members have some guesses. Top of the list ... a guy named Nick Wilson, who's currently in the final 6, has himself a longtime GF, so people think he might make the big ask.

Of course, there's also the potential of two cast members-turned lovers -- Alec Merlino and Kara Kay -- who hooked up after the show ... and broke their NDAs in the process.

'Survivor' brass banned Alec from attending the finale ... but we're told there's speculation this might be a loophole to be welcomed back in a big way.

Stay tuned ...