Each network has different engagement levels depending on the day and the time of day. Our friends at Social Media today have put together a great article on when you should post to maximize engagement. Here is a quick summary:
Thursday-Sunday between 1-4pm
The single best time to post is on a Sunday at 3 pm.
The worst day is Tuesday
Tuesday-Thursday between 10-11am
Content posted on weekends and after work tend to get less engagement
Monday-Friday between 10-11am
Content posted on weekends tend to get less engagement