xkcdexplained:

A Jewish physicist claims to not know when Christmas is because he does not “observe” that holiday. What’s happening here is a reference to an observer effect, specifically one pertaining to quantum mechanics, which states that if the outcome of an event has not been observed it exists in a state of being in all possible states at once.

It is essentially the same joke as we previously explained, here: a physicist is acting in a humorous manner because of a physics-based homographic pun.

Whatever they say — personally, I find it strange that there exists this magnificent assumption that everyone is expected to know when these holidays are — in both US and Canada, and specifically in Canada when being a graduate student, I never happen to know the existence of these holidays from anything other than the unusual store hours of the otherwise-24-hour stores… :-)