Justin and Hailey Bieber Make Wedding Guests Watch 'The Notebook'

Justin and Hailey Bieber are feeling extra sappy as they get ready to walk down the aisle ... because, before their big day, they made their wedding guests watch "The Notebook."

Sources at the Biebers' blowout South Carolina wedding bash tell TMZ ... Justin and Hailey hosted a huge sleepover for all their friends and family Sunday night at the Montage Palmetto Bluff, complete with a screening of their favorite flick, plus some fun and games.

Our sources say the evening had a carnival theme ... and there was a huge spread of ice cream and candy, a giant dance party, bowling and a big game of capture the flag.

The party lasted deep into the night ... we're told Justin and Hailey surprised everyone around 1 AM when they busted out a bunch of water guns, and a huge water fight ensued!!!

The wedding venue looks gorgeous, so just imagine running around the grounds with Super Soakers!!!

Sounds like a totally different scene from the evening's more traditional rehearsal dinner ... which featured a select group of guests hopping on boats for the fancy meal.

As we've reported ... the wedding party is crawling with celebs, including Kylie Jenner and Travis Scott, Kendall Jenner, Ed Sheeran, Jaden Smith, Usher, the Baldwin brothers and a TON of models.