In my case, I already contacted some PIs whom I want to do my rotations. I think it is better they know beforehand that you're interested in working with them for your rotations and probably for your thesis. Sometimes, they accept a certain number of students per rotation so it's better to contact them early. One of the PIs that I contacted told me that I could rotate in her lab at the start of the Fall semester. However, if you're still undecided which lab to rotate I think you will have a chance to meet all of the PIs and know their research projects. From there you can start to select. Hope it helps.