It really does pay to follow the Twitter accounts of game development companies. Sometimes it isn't a large press release, or an article, or a Facebook post that gives away a bit of information, but a tweet. Rockstar confirmed the release date of GTA Online's next DLC in under 140 characters not too long ago.
Further promoting their current set of bonuses in GTA Online, — in order to get people to spend as much in-game cash as possible prior to the DLC release wink wink — Rockstar posted the following:
The caption says it all, really. We've already theorized that Tuesday, 13th of December will be the release date of Import/Export, based on several hints. The first and most basic tell is that Rockstar pretty much always releases GTA Online DLC on a Tuesday.