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. (TV: Shada, The Five Doctors)

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.