The Harvest Moon
After you complete the game's story, the restored powers of the Harvest Goddess will trigger a special night on the 8th and 22nd of every season. Tiny will pop up in the morning to remind you that these are special Harvest Moon nights when her powers are at their greatest because of the full moon.
Sometimes a festival will also occur on the 22nd. The Big Fish Festival (Summer 22) and the Music Festival/Harvest Moon Festival (Autumn 22) will occur during the day. The full moon will still happen on those nights.
Starting at 7:00 pm, the powers of the Harvest Goddess will spread throughout Anthos. Green obs will float around the air, and a few magical things will happen. This event will continue until 6:00 am the next morning. You will be exhausted the next morning if you stay up all night to take advantage of the goddess' blessing, but it's worth it.
The first benefit is the quantity of Harvest Wisps available. All normal, pink-coat wisp will quadruple, letting you collect 4x the normal amount of seeds.
The normal Harvest Wisps that hover in the water surrounding the Harvest Goddess Spring, which you can only reach during summertime, do not split into multiple groups. These fellows will remain single during the Harvest Moon.
The blue-coat rare wisps will also be around and will be more courageous thanks to the Harvest Goddess' enhanced power. You can freely run up to any rare Harvest Wisp and collect its seeds without the wisp running away.
The abundance of seeds is beneficial for players who are working to increase their crop abilities. Doing so is required for earning higher star ranked crops and triggering giant crops.
Another benefit of the full moon is the appearance of all wild animals. Both daytime and nighttime creatures will be out in the world. Like the rare wisps, the animals will not flee when you run towards them during the Harvest Moon. Make sure to pet and give a gift to any animal you might be interested in taming in the future.
The last benefit is the affect the full moon has on your crops. Tiny will mention during the game's story that crops grow stronger when influenced by the full moon. What she is hinting at is the Harvest Moon's ability to increase the star rank of all planted crops.
The plant's star status will produce a more profitable crop. You'll find that many of your crops at harvest will be at 1 star when the plants were grown during a Harvest Moon. The star rank boost can also be beneficial for players trying to grow giant crops, as those only appear when a crop's star rank is full. If you're having a tough time getting a giant-qualified crop to supersize at harvest, try planting the seed on the 7th or 21st to take advantage of the Harvest Moon the next night.