I’m travelling to US in Christmas time. I was thinking about visiting the colleges
that I want to apply to. However, someone told me that they’re closed and I have nothing
to do there in this period.
Some colleges will physically block access to the campus over the holidays, and have private security to ensure that nobody unauthorized steps on campus.
Some colleges will allow access to campus, but most buildings will be locked. The only people who’ll be there are a handful of graduate students and faculty who are checking on experiments that can’t be left alone (such as live animals).
Some colleges will allow access to some buildings, but you still very unlikely to meet anyone.
It won’t be a very interesting visit. If the main thing you are worried about is the location and the layout of the campus (maybe you are in a wheelchair and want to check how wheelchair-friendly the walkways are?), you can visit over the holidays. If you are hoping to meet people, don’t bother.