Hypocrisy of celebrating religious holidays
This argument is most frequently used as a means of critisizing 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 adress Anyones beliefs aside from identifying what they are, and then quite possibly critisizing 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 at discrediting the opponent or making them look silly or in a bad light.
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.
So the counter argument is simple, the apologetic is unknowingly hypocritical, they are themselves celebrating the holidays of other religions.