I have checked this two Press release site randomly and found that there are still number of press release on each day including saturday and sunday, however there are more number of press release on tuesday,wednesday and thrusday as pointed by Peter, I think he has done lot of research btw.
http://prweb.com/
http://www.sanepr.com/
I think its more important that your news should be "News" and the place where are you going to release should have good PR and reader base. I hope readers might be concentrating more on press release on saturday and sunday as they are in relaxed mood and since there are less press release coverage on those days your news would be not overlapped with tons of other news, however my view is totally subjective.
