The Harvest Time event is a limited time event which only
appears now and then.
For this event, you gain points for harvesting each type of
resource and you can collect rewards after you have harvested
a specific amount of resource type.
For example you can get rewards for harvest 600k wood, 2.5m wood
and so on.
These are good opportunities to collect speed ups, hero potions, crystals
and resource packs.