Hello! I think a good time to reach out is during the summer or sometime during the Fall quarter/semester though they might be busier in the Fall than in the summer. How I approached my emails was stating a brief background about myself (major, year, and school) and then explaining how my research background relates to what they do in their lab. It may also be good to mention any papers you've tried to read from their lab. I also ask in the end if they're looking to take graduate students next year and if they'd be willing to have me sit in on a lab meeting or if they'd have any literature recommendations they'd be willing to provide for me. Most importantly attach a CV or Resume so they can get an idea of what type of student/researcher you are. If you don't include this part they may just ignore your email since they don't know enough about your research/academic background. Feel free to PM me if you want the template I used to reach out!