<p>Personally I would wait until the beginning of the semester or a couple weeks beforehand to start sending out these emails. Unless you live close to the campus, you won’t be starting until you get to school. Also it will be easier to follow up on emails in person on campus, which is often important in establishing a connection. </p>
<p>Aside from what has been mentioned here, I would also stress getting to know the graduate students in the lab some before you join. More than likely you’ll be working with/for one of them because professors often don’t have time to guide their own graduate students in the lab, let alone undergraduates. And you probably won’t get an independent project from the start, so your relationship with the graduate students is very important as they are who you’ll be working with the majority of the time.</p>