I remember reading an article about Tuesday being the best day to get maximum reach for your message. I think this study came from Harvard. The content of the message could be PR or some other communication.
Is there evidence for this?
It depends what sort of press release you are talking about. You may like to focus the question more.
For example, in a recent document, three accountancy researchers claim:
While there is considerable research on the timing of interim and annual financial reports, there is a scarcity of research on the timing of nonperiodic disclosures.
Reference: Balatbat, Maria C. A., Gallery, Gerry Thomas and Jackson, Andrew B., The Strategic Timing of Management Earnings Forecasts (February 15, 2011)
They go on to provide evidence that organisations do deliberately delay financial disclosures to hit certain days of the week.
I believe this was for a conference proceeding, and not peer-reviewed, but was based on a PhD thesis.