Update: Looks like Nudgoo in the comments has it: Maiden to confirm they’re headlining Sonisphere next year. (Blabbermouth) This was tipped back in October by Virtual Festival.
The Official Maiden site has a new banner, telling people to come back on Dec 9 at 6PM (GMT). WATCH THIS SPACE, it proclaims:

What could it be?
My guess is an announcement for shows next year, particularly summer festival dates. Classic Rock magazine is hosting a festival next year, and Maiden would be a natural fit, however they may also be too big for the show.
Another Donnington appearance? Or special one-off Clive Burr charity show?
We’ll know tomorrow night!

Sonisphere