The example you provide falls under both categories, and it falls under political correctness precisely because it would be discriminatory to celebrate one religious festival without celebrating all of them. It is not politically "correct" to partake in discrimination.
I don't know if you're from the UK, but society today is highly secular. A British Humanist Association poll last year had 65% of respondents answer the question "Are you religious?" negatively. Of those who claim to be religious, most do not attend church on a regular basis, or take part in other religious rituals. Not to mention many of them are not Christian.