After you complete Farming Tips 3, you can access a three-room cellar below your farm where you can set a season in each room.
Unlocking the Cellar
To unlock the cellar on your farm, you need to complete the three Farming Tips that Frank gives you. He will bring you the first Farming Tips on Summer 1 of Year 1, and he will bring new Farming Tips after you complete the previous ones.
The door to the cellar is in the northeast corner of your central farm area.
About the Cellar
The cellar has three rooms. The first room (with the ladder to your farm) is the center room, and there is also a room on the left and a room on the right.
Using the Cellar
The cellar comes with fields already in place. Each room has 21 fields, for a total of 42 areas to plant seeds in each room. You can't use the Fun Time Work Bench in the cellar, so you can only plant crops there that grow in fields. You can't put mushroom logs, fish hatcheries, beehives, paddy fields, or farm circles in the cellar.
There is a high-level mining node in the center room of the basement, in the northwest corner. This mining node is the same level as the one in north Westown near the post office. It requires a hammer of at least level 3 effectiveness.
Changing the Season
You can buy sun stones in the shop 99G and Up in Tsuyukusa after the cellar becomes available. You can use these in the cellar rooms to set a season in each room.
There is a bug when trying to change the season sun stones in the cellar. To avoid the bug, put all of your sun stones on the floor except the one that you want to switch the room to, then check on the altar to change the room to the stone that is in your bag. If you have multiple sun stones in your bag, the game might incorrectly change the season to something that doesn't match the sun stone that you are using, and there is also a chance that your sun stones could be changed into different seasons, so make sure to only have one sun stone in your bag when you check on the altar.