If you can swing it, go as soon as you can. Not only will you be looking for rooms with 150 other med students, but the dental, law, nursing, etc etc will also be looking. There's not really a shortage of places to stay, but it's always nice to have choices!
A lot of places are cleaning up apartments during the summer so it's usually not a problem to secure a place for August. I already went and got a place, but I'm MSTP and will be starting a lab rotation in July. However, like I said, most places didn't seem to mind reserving a place for Aug as long as you give them the security deposit.
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