At the Coolidge midnight on May 1st, according to the marquee, and the website, but you have to scroll down the events page a bit.

It is a sordid little burg, isn't it?
Ian Brennan and the Independent Film Festival of Boston present John Waters live in-person to perform his hilarious one-man show detailing his groundbreaking career in a rapid-fire stand-up comedy format, and discuss his shocking new return-to-form "A Dirty Shame" (starring Johnny Knoxville and Chris Isaak. Plus he'll conduct an audience Q & A afterwards.
(i seem to spend every May 1st at the Coolidge, for some strange reason.)