Over the past years my husband's stepmother has hosted a Christmas Eve gathering. However, as things changed the event changed. So my husband's family gets together on Christmas Eve and her family gathers on Christmas Day. We have not been extended an invitation this year but I heard through the grapevine our attendance is assumed.
I've tried several times and I'm still not sure whether you're saying that the planning this year is different than usual.
If the party is a yearly tradition that hasn't changed, then most families (in my experience) don't explicitly invite each other every year. The only reason my family ever contacted me was if the party was being hosted by another family member.
In that case, I don't think there's much going wrong here. But I somewhat assume that you're posting this question because it's not just about a recurring yearly family event that is consistently planned on the same day and in the same location.
The underlying tone of your question seems to be that there is some either passive aggressive or uncaring behavior from the other side. Regardless, I would advise you to take the high road here and remain polite since you cannot be 100% sure there was ill intent here.
Regardless of past events, never assume malice. If you jump on the malive wagon and end up having misjudged it (or your response is deemed disproportionate by third parties even if you were in the right), then you're going to have to suffer the consequences, which could even lead to you bearing the responsibility of the friction between you and them (in the public eye).
Instead, take the open and honest route. You've experienced two things that seemingly contradict one another: you've gathered that you're expected somewhere that you haven't been invited to yet (that's an important element).
Simply contact the host of the party (I presume it's the stepmother) and explain your confusion to them. Something along the lines of
Hi, I just wanted to clear something up. I heard from [X] that you're expecting us on Christmas Eve, but I wasn't aware that you had planned something for then.
Leave it at that, and let them respond. Unless there's some grudge being openly carried, odds are it'll get resolved by them acknowledging that the invitation was forgotten about, an apology (sincere or not), and you can move on with actual knowledge of the party.
Note even if their response contains a white lie, that does not prove malice. It's possible they genuinely forgot but are too embarrassed to admit it. Again, never assume malice.
If instead you receive a combative answer, that gives you an opening to not attend this party - and it won't be based on an assumption but an actual fact (their response to you).
...and let her know we do not appreciate being notified 3rd party that our attendance is expected
What you expect is reasonable. But if you phrase it like that, you come across as either combative or accusatory. This makes you lose in either case:
- If they did not have ill intentions, you've just offended someone who doesn't deserve it.
- If they did have ill intentions, you've just taken the first open shot and will be painted as the aggressor in the ensuing conflict.
In either case, you're going to be considered in the wrong by third parties who hear about this conversation. Instead, stick to being unreproachable so that things can't get turned around on you.
If you intend to attend the party but would rather not go through being forgotten again, mention how it's fortunate that you hadn't yet made other plans. If they mistakenly forgot to invite you (or put if off thinking there was no time pressure), it serves as a gentle reminder that you might no be able to attend when invited last minute. If there is bad blood here, it opens the door to not attending the next party where this happens.
If you would rather not go to the party anymore because of this, then simply inform them that you've already made other plans by now and apologize for not being able to make it - if only you had known sooner!
I tend to consider lying to be a bad IPS approach, but a polite lie to avoid a party you otherwise would dread to go to is IMHO acceptable. Telling them you've already got plans is the kinder way to say no to an invitation compared to telling them you're not attending the party even though you don't have any other plans (this would again delve into the combative territory you should try to avoid).