I was in the same position last year - while @Alice Rhyl's answer is correct, and there is nothing technically wrong with doing this, it is important to be clear about your intentions when talking to the CBP, especially if it's less than a month before your program start date (although since you're planning to travel in May you should be fine).
In particular, my university's international student office advised against doing this because it can cause issues if you are accidentally admitted with F-1 status rather than under your ESTA. It is important to make clear to the CBP that you are intending to enter as a tourist, because if it's less than 30 days until your program start date, they are allowed to admit you with F-1 status. You do not want them to do this if you are planning to return to your home country before your program starts. The main issue is that the first time you are admitted on F-1 status, you can't leave the US before school registration and your SEVIS record is updated, or it may become very difficult to reenter.
See for instance the advice here under 'Travel After Arrival to the U.S'.
(Also please don't attempt to mislead CBP about whether you're visiting your partner or just a friend as suggested in the comments, it isn't worth the potential trouble. Anecdotally, I entered the US with an ESTA 13 times over a two year period to visit my partner before I entered as a student, was honest about it every time, and never had a problem. This is a common, and good, reason to visit as a tourist!)