The Last Day comes in at 3 minutes and 57 seconds long, and will be included on the DVD release of The Day of the Doctor. Further details, such as who is starring in the extra (it is rumoured that Paul McGann may be starring in a special 'mini-sode', which could be The Last Day) are yet to be confirmed.
It's likely that we'll hear details soon with an online debut before the anniversary special itself.
The Day of the Doctor is released on DVD, Blu-Ray and 3D Blu-Ray on Monday 2nd December.
[Source: Blogtor Who]