A thread below asked about the Globe guide. This is an unbelievable overview of festivals for the summer. It is also worth looking at the list she prepared for 2001:
Although the present year has far more listings, there are a few festivals in the 2001 which didn't make 2003. I doubt this means they no longer exist - I have added them to the new list to make a bigger-than-both "uberlist" (with the caveat being: check before going to anything).
And yes, I have searched extensively for any 2002 list. Doesn't seem to exist.
(PS: For the specific case of Greek festivals, there are other sites like GreekBoston and Hellenic-something, which have far more complete lists. I learned the hard way: last week that Arlington festival was nothing compared to the grandiose one happening in nearby Woburn at the same time.)