The Radio Times (who initially reported that the Twelfth Doctor would be announced in July), have claimed that the Twelfth Doctor will now be announced in August or September. They claim:
BBC insiders have told RadioTimes.com that filming on the Christmas special – to be followed by the full series – will not take place until the autumn – probably August, or even September.
According to sources the identity of the Twelfth Doctor is almost certain to be announced just prior to filming, which means that it is very likely that the new incarnation of the Time Lord will not be unveiled until late summer.
Initially the BBC had hoped to begin filming the Christmas special earlier, with some sources suggesting July as a potential date.
“Suffice to say we are completely on track,” said a spokeswoman.[Source: Doctor Who TV]