Between 7pm and 9pm, any day of the week. That is when the largest number of users are active, which gives a post its best chance of hitting an early view peak. It is a global average sourced from Instagram's own data, so treat it as a starting point rather than a rule.
Because the window in which a post performs has narrowed. Around 76 percent of a post's total views arrive in the first three days, and the steepest part of that curve is day one. Publishing into a quiet hour does not delay your reach, it reduces it, because the early engagement that would have signalled quality to the algorithm never happens.
It is wrong whenever your audience is not the global average, which is often.
Use the 7pm to 9pm window as a default until you have 20 posts of your own data, then trust your own numbers over anyone else's.
The best posting time is one where you are actually available afterwards. Comments arriving in the first hour compound, because the algorithm reads early conversation as a quality signal and distributes accordingly. A perfectly timed post published before you disappear for the evening is a half used post.
Pick your next five posts and publish them in the 7pm to 9pm window. Stay available for 30 minutes afterwards. Then compare their first two hour performance against your previous five. That is the only timing study that matters for your account.