If one reads the article by Tori Sabourin in a Ragan article published on 23 November 2015 there is no real proof that there is a best time of day or day of the week although in an analysis of 100 000 press releases it seems that Tuesday is the best day of the week, followed by Wednesday. The problem comes when the press releases are shared or not shared. It seems that the top 10 most shared press releases represent 14 percent of total social media shares. That means 0.009 percent of all releases got 14 percent of total shares.
This is just more evidence that if you have interesting shareable content that is more likely to be shared on social media than your corporate media release.