I really don't know because I was only an intern, but I guess they took vacation days (law mandates minimum 20/year). But I also recall a discussion here on HN on this topic, where applicants (at a different company) were in invited to work on a weekend.
2 days is probably overkill for US-companies, but a bit of pair-programming on real problems seems a lot better than "Let's see if you memorized this trivia fact you'll never need in your job"
How is this even possible for people that already have a job? Are they supposed to take two days vacation or do it in the weekend?