Derek Jeter -- Huge Party Planned At NY Palace ... Possible Secret Wedding?

Derek Jeter will be front and center at a MASSIVE secret party at a historic castle on Long Island this weekend, TMZ Sports has learned ... and now we're hearing buzz it could be a wedding.

Here's what we know ... the party is set to go down at the famous Oheka Castle -- which was used for the exterior shots of Charles Kane's mansion in the 1941 Hollywood flick "Citizen Kane."

Multiple sources tell us ... several high-end party vendors have been called in for a very important "Derek Jeter event."

Now here's where things get tricky -- we've spoken with multiple sources who tell us the event in question is a retirement party for #2 ... and other sources who SWEAR it's a wedding.

Of course, 40-year-old Derek has been dating 24-year-old supermodel Hannah Davis for the past couple of years ... but if they got engaged, they've done a helluva job keeping it a secret.

We reached out to Derek's people for comment -- so far, no word back.