Yes, there is a best time to post, and it matters less than you think. The best time for your account is when your own followers are on their phones. There is no best time for everyone, and the charts online are averages of accounts that are not yours.
Where to find yours
Instagram shows a professional account when its followers are most active, by day and by hour, in the insights section. Facebook shows the same for a page. Look once. Pick the two slots that fit your posting days, a Tuesday evening and a Friday morning say, and stop thinking about it. A café will find its followers scroll at breakfast and after nine; a furniture maker may find Sunday afternoon. If the account is new and there is no data yet, post when you would read: after the school run, or in the evening.
What matters more than the hour
Steadiness. A post at the perfect hour on an account that has been quiet for a month does nothing. A post at an ordinary hour on an account that posts every Tuesday and Friday does its job, because the people who like the brand know when to expect it and the account looks alive to a stranger. Most questions about timing are really questions about rhythm.
The best time to post is the time you will actually post.
The exception
Anything happening now. A market stall this morning, a launch at noon, a sale ending tonight, the last three of something. Post those when the thing is happening, not when the chart says. The chart is for the posts that could go out any day.
Stories and posts are different
A post sits on your profile and works for weeks, so its hour matters least. A story is gone in a day, so it wants the busy hour more. If you only look at the insights once, use them for stories.
Pick, then forget
Two slots, fixed, written into the plan. Revisit them in six months, when the account has grown and the followers have changed. Between now and then the hour is one fewer decision per post, which is the point.
What CP3® does here
Drafts land on your calendar with a date. Move any post to the day and slot that suits you, approve it, and it is scheduled. Instagram publishes from the calendar at the time you set. Other channels you post from the draft when the slot comes.