Here are the two prequels we were promised before the 50th. Both excellent in their own way. The first one, "The Night of The Doctor," sets the stage for John Hurt, finalizing where he fits in with the other Doctors, while "The Last Day" gives us a glimpse into The War Doctor's first day in the time war. Someone said it looks almost like a setup for a game about the war, and I can see that.
The Night of The Doctor
The Last Day
You will notice I did not say SPOILERS. That's because these AREN'T spoilers, they are prequels meant to be watched and to give us some information, most likely background, to prepare us for the 50th.