There was a presale on 15 December for members of the 2CellosItalia Fan Club. You can subscribe to their newsletter here: 2cellositalia.it/
[Original Announcement: There have been 2 short video clips by Luka and Stjepan finally announcing that SOMETHING is happening to celebrate the 5th Anniversary of the release of Smooth Criminal.
Speculation is rampant that it will be a concert in Italy because the video clips were orginally released through the Facebook of one of the 2CELLOS_Italia fangroups and one of the clips is sort of in Italian, although Stjepan and Luka seem to take great pride in seeing how badly they can mangle the language. Links to the clips are below.
There have also been suggestions that it might be a concert in Japan, since Japan frequently gets the "best" stuff first, or even that it might be some sort of online broadcast/performance so that all their fans everywhere in the world can enjoy it.
More information is due soon and in the meantime, the Italian fan group has set up a page with a placeholder date of 1 January 2016 to be updated as soon as more is known: www.facebook.com/events/1498588483775735/