BBC Media Centre has released the official synopsis of the 50th special, The Day of the Doctor.
In 2013, something terrible is awakening in London’s National Gallery; in 1562, a murderous plot is afoot in Elizabethan England; and somewhere in space an ancient battle reaches its devastating conclusion. All of reality is at stake as the Doctor’s own dangerous past comes back to haunt him.
Given what we know about the special (e.g. the leaked synopsis and the Dalek images), we could presume the "ancient battle" is the end of the Time War on Gallifrey and the "murderous plot" part could simply be the Zygons and the Tenth Doctor. You can also see pictures and videos from the filming outside of the National Gallery here and here.
The exact time of the special is still to be confirmed.