Depends on your market and their time zones if targetting a different country or multiples. You may need to post at 8am their time but it could be 5pm your time. You need to do some research into what they're doing. If you're targetting stay at home mothers, you won't want to be posting when they're getting kids ready for school. If you're targetting workers, then commuting times are normally good. High school students? after school or on their lunch break.
I'm targeting the UK and Japan, so I try to post at times where I am reaching them both at good times. Doing that for 2 times is easy enough but the 3rd time normally end sup being middle of the night in the UK. I try to aim for when they will be going to work and school, especially for Japan where a lot of people use trains (so they're on their phones).
Once you've been posting for a while, try a keyhole.io trial for 3 days to check when it's the best time to post.