May Week

May Week was a period of time, actually taking place in June, to which the Fourth Doctor intended to take Romana II. The TARDIS, however, was confused and instead took the two to October. (HOMEVID: Shada, TV: The Five Doctors)

Liz Shaw had earlier asked the Third Doctor to come to Cambridge for May Week. This confused the Doctor, who said it was June, but Liz clarified that May Week is held in June. (PROSE: The Scales of Injustice)

Behind the scenes
In the real world, May Week did originally in fact take place in May. However, the event nowadays not only takes place in June, but is also ironically two weeks long.