Ibn al-Qayyim (رحمه الله) said: "As for congratulations/greetings for the ceremonies of disbelief that are specific to it, then it is unlawful by unanimous agreement. Such as greeting them on their festivals and days of fasting by saying, 'A blessed celebration upon you'and 'Greetings on this celebration' and its likes. Though the one who says is safe from [having fallen into ] disbelief, it is [still] from the unlawful things. It is the same as congratulating him for prostrating to the cross."
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