Keep in mind that if this is a large organization and your manager and the project manager/leader is from a different group that the project manager or team may not appreciate you taking time off immediately like this. In my org (a very large firm) this sort of thing would be frowned upon for a new hire (not from HR or the manager, but the project team would notice for sure).

So if I were you I would consider the following.

1. Immediately ask the HR person what the accrual policy is and that you would like to take a vacation around the time of Thanksgiving.

2. Depending on the answer (most likely there is an HR loophole for the time-off) then discuss it with the manager and specifically focus on what projects you will be working on and ask how much time is typically billed during the thanksgiving break (you may be assigned to a client who is barbaric/efficient and has folks working the Friday after thanksgiving).

3. If your manager says that Thanksgiving is a dead week and HR says you can take vacation accrual in advance then broach the subject of taking part of that week off. Keep in mind that you must be back and ready to go at the client site when the work week starts after thanksgiving, not tuesday or so.

And as others have said, if you are going to be focused on your career you should most likely skip the vacation and rebook for dates that you know will be free (you will figure it out in the first two weeks) you should find a more experienced peer and talk to them about expectations etc.

If you end up eating the stay with hotwire then you should consider it a costly lesson but one that you won't forget.

Best of luck in your new career and remember to work hard, but also work smart and stay on the happy side of revenue.