mass going on xmas and athisism.

Commander Vimes wrote:

yes and that family tradition is something that he has strong views against.
what if that family tradition was to pray to the moon goddess? would you go and wear the robe and do the chants and draw the pentagram in your own blood considering your strong belief in the catholic church?

Having been to hundreds of rites in honour of moon goddesses I have to say I've never seen that done. For that matter it takes quite a bit of blood to draw anything sizable, so unless it was a rather small pentagram you are likely to need a lot of blood so unless one had very heavy periods (the only human blood with any particular lunar significance) it wouldn't really work.

That said, if there was some hypothetical religion with such rites, why not go as a guess and not do the drawing bit?

I went to an Anglican service this Christmas and it was quite lovely. However I was certainly not expected to go by my girlfriend or her family but invited by them. Now there's sometimes quite a grey area as to how much someone invited to anything is expected to accept that invitation, but it's still a completely different kettle of fish to be invited rather than expected. Neither my girlfriend nor her family are under any sort of illusion that my joining them in a rite that is important to them makes me a good Anglican; it makes me nothing more than a guest of their family, and one grateful for their hospitality at that.

Now the OP is in the position where they are expected but they can turn that to a position of being invited simply by saying they will attend the service if someone invites them to do so.

Their parents will probably immediately invite them, and they can then go along, join in the rite to the extent appropriate for a guest (e.g. not only would you clearly not pray or partake in a sacrement such as the Eucharist since it is not of your religion, but from the RC point of view a guest is welcome in the church but not allowed to take the Eucharist).

In other words draw the line between your beliefs and your parents but then go out of your way as to go as close to them as that line allows. Do that with continual good humour and continual good living and all but a fanatic will eventually come to respect your position. A more pointed refusal may be appropriate in the wider context of a fight over the relationship between Church and State but will only breed bad feeling and leave to your parents respecting your position less rather than more.