What do a priest and a Christmas tree have in common? They both like fairies sitting on them.
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Explanation
The joke uses the word "fairies" in a double meaning. The first meaning refers to the small, winged mythical creatures which may adorn a Christmas tree. The second meaning is a slightly old-fashioned slang term for a homosexual man, suggesting that both a priest and a Christmas tree enjoy having a gay man "sitting on them." This play on words is what makes the joke funny.