Actually, if you send out press releases via a free wire service (PRWeb), that is when you have to wait for it to post or 'release'. Paid services (PRNewswire, MarketWire, Businesswire, etc,) send out when you want them to be delivered. There is no rule for the best days, however most people send on Tuesday mornings. I never recommend a Monday, Friday or weekend distribution unless it is 'breaking news'.