Easter

On the first Easter, Jesus Christ was killed by crucifixion. (PROSE: Byzantium!)

Because it was always at a different time in the year, the Doctor had difficulty navigating his TARDIS to the correct date. (TV: Planet of the Dead)

History
The Tenth Doctor claimed to have been present at the original Easter, and intimated that the events were at variance with what humans believed. (TV: Planet of the Dead)

On Easter of 1996, seven-year-old Amelia Pond was praying to Santa Claus. She asked him to send someone to fix the crack in her bedroom wall just before the newly-regenerated Eleventh Doctor crashed in her garden on Earth and met her for the first time. (TV: The Eleventh Hour)

On Easter of 2009, Christina de Souza stole the Cup of Athelstan from the International Gallery in London. She then met the Tenth Doctor, who had managed to arrive on Easter and was enjoying a chocolate Easter egg, on the 200 bus in her escape. The 200 drove through a wormhole to the planet San Helios. With the help of the Tritovores as well as UNIT, those on the bus made it back safely to Earth, the wormhole was closed by Malcolm Taylor, and all stingrays that had made it through as well were shot down. Christina escaped the police in the 200, with the Doctor's assistance. (TV: Planet of the Dead)

Other references
Reverend Tonge was eager to have Fenella Wibbsey for the Hexford Christmas show, because, according to him, "everyone was enthralled at [her] dancing the can-can during the Easter parade." (AUDIO: The Hexford Invasion) Easter Parade was a film as well, starring Fred Astaire. (TV: Something Borrowed)

In a shared dream, the Twelfth Doctor coldly wished Santa Claus a happy Easter. Later, when Ashley Carter asked who Santa was, the Doctor sarcastically gave the options of tooth fairy and Easter Bunny. In a dream of Clara's set on Christmas Day, Danny Pink retorted in reply to her question, that it was Easter, and he was dressed in a Santa suit because he was "Father Easter". (TV: Last Christmas)

In the town of Christmas on Trenzalore, the Eleventh Doctor responded to Clara Oswald's question, "How can a town be called Christmas?" with, "How can an island be called Easter?" (TV: The Time of the Doctor)

Behind the scenes
Though not clarified in The Time of the Doctor, Easter Island was named as such because it was discovered by European explorers on Easter Sunday.