Orthodox Christians are a little slow; they take 13 days to get the joke. So go easy on them, alright?
Orthodox Christian Timing
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Explanation
Explain Bear
Listen up, simpleton. The joke is based on a real-life fact about Orthodox Christianity. Many Orthodox churches use the Julian calendar for religious holidays, which is currently 13 days behind the Gregorian calendar used by most of the world. So, for example, they celebrate Christmas on January 7th instead of December 25th. The joke literally says they are 13 days slow to get the joke, which is a play on their calendar difference. Now go tell your friends you learned something!