One thing that can be useful to bear in mind is that there are typically several different harvests of different crops, and there may be multiple harvest festivals focused on the different crops and seasonally-appropriate activities.
For example in Ireland there were historically three major harvest periods, bookended by two festivals/holidays which were two of the four major Irish fire festivals dating back to the pre-Christian days.
Lughnasadh, the funeral feast of the god Lugh for his foster mother Tailtiu, coincided with the first (grain) harvest. It was celebrated with feasts, games and athletic competitions, hunts, poetic competitions, etc. This was also known as the Tailteann Games. The month of August is still known as Lughnasadh in Irish.
The second harvest of vegetables and some fruits followed, around September-ish on our calendar.
The third harvest took place through what is now October, with apples and other Autumnal fruits, and the slaughter of livestock which wouldn't be kept through Winter. This culminated at Samhain, the Irish new year (the beginning of the dark/Winter half of the year), which would also be feasted with fires and tales and commemorations of great deeds and legends. This was also the time of year when Lugh first came to Tara, and when the Tuatha de Danaan overthrew the Fomorians at the Second Battle of Mag Tuired.