Easter
From Tardis Wiki, the free Doctor Who reference
Easter was an Earth holiday. Because it was a moveable feast. the Doctor had difficulty navigating his TARDIS to the correct date. The Tenth Doctor, however, was able to land on Easter 2010 in time enjoy a chocolate Easter egg and to save the Earth from an incursion of Alien Stingrays. On this occasion, he also stated that he had been present at the original Easter and intimated that the events were at variance with what humans believed. (DW: Planet of the Dead.
