Britney Spears will finally show off her engagement ring TONIGHT -- but before it goes public, she's holding a private gathering in Vegas to give her friends and family the first peek ... TMZ has learned.

Sources close to the couple tell TMZ ... the intimate event will take place at the Planet Hollywood Resort and Casino BEFORE Jason Trawick's big celebratory dinner.

We're told the owner of the PH -- Robert Earl -- will personally attend the event ... and plans to make a toast to the happy couple.

As TMZ first reported, the pair will then move on to a MASSIVE celebratory dinner at the Chocolate Lounge at the Sugar Factory in the Paris Hotel ... followed by a dance party at Chateau nightclub.