Underneath The Sithmas Tree.

Darth Vader and Luke Skywalker are having a lightsaber battle on the Death Star, when Vader says to Luke:

"I know what you're getting for Christmas."

Luke is somewhat bewildered: "It's not true, that's impossible!" so Vader again says to Luke: " I know what you're getting for Christmas."

Luke replies, "How in the Name of The Force would you know that?!"

Vader sighs and finally explains: " I felt your presence."

Sorry, it's been a long day.