I've always really enjoyed the first day of a class, because of the opportunity it represents to engage with a new and unique student group. The preparation, to-do lists, meet and greet are all great, but it's the learning that I do that I think best sets up how the rest of the class will run.
It's a process of gauging student involvement, building rapport, perhaps challenging a few students who don't seem to be quite invested in the class--but also to challenge my own first impressions of students, almost like giving them a second chance at it.
Trevor