I'm a postdoc at the ETH, and my understanding is as follows:
You will have 5 weeks paid holidays. In addition, the ETH is closed for one week between Christmas and New Years. Your advisor can ask you to work extra hours to make up for this time (which is standard policy) or can give it to you as a courtesy. This is in addition to cantonal holidays, though in my institute grad students will work these days (as do postdocs without families).
As a student, you may get a salary that is less than 100%, which is supposed to reflect the time that you spend on coursework (though, since this is a department-specific percentage, I would guess that it is more dependent on funds than anything else). However, if you're getting paid 60%, you only take 60% of a week off, so it doesn't actually affect your holidays (though don't expect to earn extra time for the extra hours you put in as a graduate student).
In addition, all vacation time must be approved by your advisor, though I have not heard of this being a problem. Paperwork is important to some groups, as accident insurance is paid by the ETH and if you get injured skiing while you're supposed to be at work, for example, the paperwork is more interesting. Also, at least in my institute, we sign an agreement not to be paid for vacation time not taken. This will, of course, vary by institute or department.
Finally, one bit of advice: those prospective graduate students who are really interested in vacation time often are not perceived in a positive light. It's not fair, but it's true. So, be discreet, and ask, "so, what do you do in your free time? Do you travel?" rather than "how many days off do we get?"
Source here:
http://www.pa.ethz.ch/rechtliches/23...oschuere_e.pdf
Page 10 is the relevant starting point.