What I have noticed is many using social media like Facebook or twitter make is a common mistake of placing their advertisements and articles at the wrong dates and times. The point is not be post when it's convenient for the shop owner but when actually the customers of the shop will most likely to see the information. What is the point of posting if by the time their customers start to look, there are hundreds of items ahead of them, their message is lost and the opportunity, the effort and money they put into it is lost!
A useful trick is to examine similar companies to yours and see what they do. That will give you an idea.
Then once you get going you need to use some of the tools available to you to determine the correct days and times. This is not so easy to determine as everyone’s best times to advertise can be different. What you need to do is experiments to try to find the correct time.