@Akili No, you've got it right
Let me try and think about what I love about holidays like Halloween despite being an introvert:
As you said: decorating! I love trying to make my surroundings fit a theme or a mood, so that's a big part of the fun for me. Also seeing other peoples' decorations, eg. driving through the streets at night in December to look for Christmas lights. It sets up a nice atmosphere that I like being in. Seeing other people socialising and having a good time is also a nice atmosphere to be in. If you see me at a party, for example, it's nice to have the company of my friends (not necessarily socialise) and see them having a good time. I like watching horror movies, maybe go on a walk in the middle of the night with a friend/family member etc.
Soo, to sum up my thoughts: I think I enjoy doing things by myself, or even with one other person, but it's the atmosphere of everyone else celebrating that I like too