When is honey harvested? The short answer is when there is honey to harvest! Bees are not like cows, where you milk them at specific times based on a schedule.
The amount of time the bees can be out collecting nectar is really important. Bees need light (mostly) to be able to navigate to flowers. This means that when the Sun is down the bees will not collect nectar.
If the temperature is right, and the days are long, we may have a situation where the bees do not have enough workers. It is important to select genetics suited to your climate.
So when is honey harvested? When there is honey in more supply than that needed by your bees. Honey in a bottle on your shelf or in a shop is worth more to you than honey taking up space in a hive.