This feature length special is a major milestone in the history of the show. It marks the Centenary anniversary of the BBC, it is the 300th televised Doctor Who story, and it's the penultimate broadcast leading up to the 60th Anniversary special(s).
This episode is part of a series of events in different shows to mark the the 100th anniversary celebration of the BBC.
Jodie Whittaker was pregnant with her second child while filming the episode. Even though she was very sick, she didn't tell anyone. She didn't even tell it to her co-star and close friend Mandip Gill, because she didn't want to worry her.
The Master mentions "Forced regeneration", a Time Lord penalty for breaking Galifreyan laws. This was a reference to The War Games: Episode Ten (1969).
At 87 minutes, this is the second longest installment of Doctor Who made for television. The longest remains Doctor Who: The Movie (1996), starring Paul McGann as the 8th Doctor. This was also the first time that Sylvester McCoy appeared in "Dr. Who" since his appearance in the 1996 TV movie.