Harvard and Yale, in their infinite wisdom and love of tradition, do not register students for classes until the second week of the semester. The first week, called "Shopping Period," in theory gives students a chance to try out different classes. Undergraduates seem to like it.
Shopping Period has several consequences. One is that no schedules can be set in stone until the second -- or often third -- week of the semester. Since nobody knows how many students are going to take a class, much less which students, graduate student-led course sections can't be scheduled until the second week. This makes it difficult for undergraduate research assistants to set their lab schedules until the second or third week of the semester. The same goes for any regular meetings that graduate students and/or undergraduate students attend. Also, one can't reserve rooms for those meetings until after all the courses have rooms assigned and all the sections have rooms assigned. I have a mandatory meeting for my undergraduate research assistants. As should be clear from above, it's not always clear whether an undergraduate will be able to attend that meeting until the third week of the semester.
Again, the students seem to like Shopping Period, so it clearly has its benefits, but it makes the beginning of the semester very busy -- it's a little like packing your bags while you're already on the way to the airport. I am also giving a talk next Thursday and submitting a paper to the Cognitive Science Society annual meeting next Saturday.