King of Avalon almost always has one or two seasonal events being active. At this time the Harvest Moon Event is one of them. This guide explains how the event works.
You have a chance at getting a Harvest Crystals when gathering resources on the kingdom map. These Harvest Crystals can be used to give to Equinox Riders who are randomly located on the map, in return for which you will get a Harvest Chest, which you can use for rewards.
This is quite similar to how the Midnight Sun event worked with the difference that during the Midnight Sun event you received the goodies for slaying monsters.
So far I have gotten 3 Harvest Crystals for depleting a lvl 7 resource tile and 1 for depleting a lvl 6 resource tile, but likely there is some randomness in the amount you get, as we have seen with previous events.
On a side note, with all the potential rewards for gathering on the kingdom map vs gathering in an alliance resource building (chances at getting materials, prestige banners and special event items like the harvest crystal), I wonder if the increased resource gathering rate of alliance buildings is still making enough of a difference.
There are Equinox Riders on the map during this event which you can visit if you have a Harvest Crystal. If you do so you get a Harvest Chest in return via a mail (neeed to Collect before it goes to your inventory).
So far I have received the following prizes from Using the Harvest Chests: Intensity Crystal (I), 100 Lord EXP, 100 Dragon EXP, 30m VIP, 10 VIP Points, 100 Dragon Spirit EXP, 1K Food, 1K Wood, 200 Iron, 50 Silver, 2h Firm Defense, Cerussite, Bronze, Fluorite, Sentinel Gem (I).
It's not much different from other recent events, and as with the Midnight Sun event, this having to exchange your Harvest Crystals for Harvest Chests gets boring very fast.