Hi there and congrats on your offer!
The question is valid. Broadly, you are going to experience two aspects of your role - techinical and people. Put in other words, you are going to show your general intelligence and technical skills, as well as emotional intelligence and people skills. There are some ways you can prepare for both:
1. Emotional intelligence/people skills (probably more important!)
* Read books (the genre is self-help, however bad it sounds) that address areas you feel you could get stronger on; "How to win friends and influence people" is one of the classics that is actually recommended by Bain as a pre-read
* Research stress management techniques, such as meditation (or prayer if you're religious). Build this into your daily schedule already today. You relaxing now is the right thing to do, but the key is to make this continue once you are on the job and things are getting out of control
2. Technical skills
* Excel is probably your best bet. Take an online course if you can. Other than that, everything you will do will be very highly specific to the situation so you can't prepare for this.
Hope this helps!
Good luck on your day one.