THIRD UPDATE: The announcement show now has a trailer. Original story follows.
On the BBC at 7pm this Sunday night we will finally find out who will play the twelfth doctor, in a live show hosted by Zoe Ball. For our American readers who might want to ensure they're near a computer at that time that's 7pm UK time, which is 11am PDT and 2pm EDT.
Matt Smith and Steven Moffat will join Ball on the show with the man or woman who will be playing the new Doctor.
Casting was supposedly done under the code name Whodini and even those working with the twelfth doctor were not aware that that was what they were doing.
The BBC One schedule for this Sunday currently has an episode of Celebrity Mastermind scheduled for that time. I presume that's getting bumped for this special.
UPDATE: MSN UK Entertainment have just Tweeted the following regarding a Doctor Who announcement at midnight tonight,
#DoctorWho fans. There will be an announcement at midnight. That's all I'm saying…
— MSN UK Entertainment (@msnents) August 1, 2013
Don't get too excited though as we believe that the announcement they're talking about will actually just be about the live show and not any announcement as to who the new Doctor will be. The BBC will be saving that for Sunday.
SECOND UPDATE: We now have some more details of the show.