Story related - Can't be missed
You will be given a hoe & watering can by Jeanne and the next day once you leave your house there will be a Harvest Sprite outside.
Once you've spoken to the sprite pull up the weeds and there will be a stone underneath it. Go to Jeannes house near the lighthouse shown in the image below and speak to her. You will then go to the lighthouse and are able to place the first stone tablet.