When is the ideal time to contact potential PhD advisors? I'm a rising senior studying Psychology, and will be applying to PhD and Master's programs in the Fall. I have already shortlisted the advisors from the schools I would like to work with and whose research I find fascinating. 

Would sending them a quick email now (late May) asking whether they are taking graduate students and expressing an interest in being a part of their lab in 2019 be too early? Should I wait until the term starts in September/October to reach out to them?
Assuming you’re talking about the US, the summer would be a good time to start contacting faculty about your interest, although the exact amount of funding and the availability of open positions more than a year ahead of time is murky at best. So you’re not likely to get a commitment at this time, but you will make yourself known to the research group, which could be of benefit in getting an admissions offer.