Volcano Day

The Tenth Doctor, The Fires of Pompeii
'Volcano Day' is another of those sneaky little repeated lines that crop up a couple of times in Doctor Who, especially the regenerated 2005+ series'. One of Captain Jack's lines in The Empty Child/The Doctor Dances, was, "But you'd better set your alarm for Volcano Day!"

And, fittingly enough, in The Fires of Pompeii, The Doctor and Donna arrive in the historic city to find tremors shaking the community and the Doctor realises, in one of the best cliffhangers from Series 4, "We're in Pompeii, and it's Volcano Day!" Cue title sequence!