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Timing depends on the industry, but Sunday nights are generally a good time for general releases so the release reaches journalists on Monday morning. But you need to bear in mind whether your target goes to press daily, weekly or monthly and work accordingly.
Only put out a press release when there is something important to say - ie something newsworthy.
Don't make the mistake of pumping out releases full of puff and fluff, and remember put the important news in the first paragraph. Don't bury it so the reader gives up before they get to the point!
MODS - please remove this reference if against TOS - PR Newswire is a good option, free and paid-for options available and covers a wide range of industries.
Hope that helps.
Timing really depends on your niche and what works best for your audience. Business news tends to be submitted so that it's available early Monday morning.
All other press releases (entertainment, sports, travel, etc.) seem to vary throughout the week.
You may have to send a few over the next couple of months and determine what works best...(gives you the best ROI)