@Descant, fair enough, but plenty of people answered thinking that if the OP has checked and double checked that the invite was genuine, then one would assume that it is. If it wasn't a genuine invite then that is poor form from the colleague, and she should have asked in the first place and pressed the invite, specifically saying to OP that she wasn't just doing it to be nice.
And personally I just do not get this "you were difficult, you should have accepted as offered or said no" mentality. The colleague hadn't specified exactly what she was expecting the op to do attendance wise (timings etc). The op emailed to accept the invite and start a conversation about the plans. That seems like normal behaviour to me, and I'm sure if how most people would have replied. The op explained in her email that she was thinking of the colleague and not wanting to encroach on family time, as she knows that is precious this year. How anyone is reading that an jumping to "you were rude and difficult" is honestly beyond me! Whether the op has specifically said on this thread whether she wanted to go or not doesn't seem all that relevant. She accepted the invite in good faith and tried to arrange it. The colleague chose to ignore that. That is poor form on their behalf.
I think it's a very normal response, so feel a bit reluctant to say yes to a Christmas invite with someone you don't know all that well, as you worry you will be encroaching on their family time. Her friend helped encourage her to day yes if she wanted to, which she decided to do.
The colleague is a grown adult. She invited the op for Christmas. If something about this one email accepting, offended her so badly she decided to blank the op and deem her to be unforgivably rude (as some seem to be suggesting), then she's weird and lacking in common sense and social skills herself. Ffs, if it's not convenient, reply saying so. It's not difficult. The other option just leads to mega awkwardness down the road when they go back to work.
As I've said, personally I think the colleague either didn't see it, or something has happened. If not and for whatever reason she did choose to not respond, then she's a bit of a dick.
The op emailed in good faith. She's not done anything wrong or been rude with her intent, and any reasonable person would realise that when they got the email.
I just think the constant unpicking of details is doing the op no favours, she doesn't have to justify herself on here if people are intent on picking her actions apart. It's often the way on MN. Things can't just be common sense, there always has to be some big deeper meaning to a simple interaction, which in the real world just isn't there.