Lookups spiked on December 21, 2011.
During the holiday season, Yuletide frequently appears in greetings and advertisements as one of the few synonyms of Christmas or Christmastime.
People are drawn to look it up partly because the word has a mysterious quality ("Yule?") that doesn't obviously suggest "Christmas."
In fact, it didn't always suggest "Christmas." Yuletide originates in an Old Norse word for a Pagan midwinter festival associated with the solstice.