The Harvest Festival is a week-long Event that happens usually on September or October, around the actual date of the Lunar Mid-Autumn Festival, also known as the Mooncake Festival which is an annual event, celebrated across Asian countries like China, Korea, Singapore, and Malaysia.
The date of Harvest Festival is always located on "Mid-Autumn Festival". You could search the dates of Mid-Autumn Festival on google, and you can get the exact date of when Harvest Festival is going to start.