Yes I thought that too. She just likes that day, I like the day when seasons change too. And for the release date I don't think that she was able to choose it from any other day (or at least tuesday) in the year. Maybe her label was like June 14 or June 21? And she said let's release it June 21st, it's summer equinox. But it probably doesn't have any symbolism.
Okay, my hopes for having Best Behaviour on the record are pretty much dead.