When using Instagram and Reddit, your goal is to post content that your intended audience will actually see. The best way to do that is to make sure that you are posting at times when your target audience is the most active on these social sites.
On Instgram, its general consensus regarding the best time to post is between the hours of 11am and 1 pm, as well as 7pm and 9pm.
On Reddit, the optimal posting time is around 9am Eastern Standard Time, as well as 1pm Eastern Standard Time. Doing so will help ensure your posts remain near the top of a subreddit.