Yes, I said it. Weekends usually are the worst days for Instagram, so please do not waste your best posts from Friday afternoon until Monday morning. I used to think that Sunday evening Instagram started doing better, but it has proven to be a false belief time and time again.
I generally try to hold off on my best posts for Monday through Wednesday. After that, I see a decline in user engagement.
Simple reasons that come to mind are that people are busy living their lives over the weekend. As glued as we all think that others are to their phones, on weekends, people still tend to socialize and do other fun stuff than just sit on the gram double tapping the screen. Sometimes a post that is made in the morning of one of the weekend days can still do ok, however once it gets later in the day, do not expect good engagement whatsoever.