"I decided to ask some experts about this topic via Twitter and received some helpful responses from the news wires. PRNewswire suggests sending news in the middle of the week and to steer clear of Mondays and Fridays. Business Wire on the other hand suggests early in the day, early in the week for most press releases. readMedia, a social media release start-up, sees little difference in weekdays, but advises against sending releases on weekends for obvious reasons."
I have been invested in companies which were about to release big news. For some reason, some of those companies chose to release important PR's or have a CC late in the day on Fridays.
I think it gives the market time to analyze the results over the weekend. Then, those stocks really went up the following Monday and throughout that week.