Hypocrisy of celebrating religious holidays
This argument is most frequently used as a means of criticizing atheists, or perhaps those of other faiths than their own, for celebrating a holiday not of their faith (or no faith). The argument does not seem to address anyone's actual beliefs aside from identifying what they are, and then quite possibly criticizing the person, so in that sense the argument is an ad hominem. It doesn't have much of a purpose in an actual argument or debate and usually leads to a pointless attempt to discredi the opponent or make them look silly or in a bad light.
Counter-apologetics
It may also be useful to realise that, for atheists, the holiday in question is not celebrated for its religious values and should not be considered hypocritical on any level. This is probably true for most other faiths although there is most likely some variation, the same holiday may be celebrated for different reasons by different Cultures or Religions.
Also, most holidays have been co-opted by newer religions which they then claim as their own. Both Christmas (see The sun in religion) and Easter are old pagan celebrations that Christianity claimed.
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Another thing to point out is what people do on a Thursday. Thursday was originally the celebration of the Norse God Thor, the god of thunder. Every day of the week is a celebration of some sort to a Roman, Norse, or pagan god. There is a similarity for the origins of names for certain months.
So the counter argument is simple, the apologetic is unknowingly hypocritical, they are themselves celebrating the holidays of other religions.
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