In the back of the Sprite Company Tree you'll find the Casino. The casino will open after you have found the pit-boss Sprite, Roller. After the 8th of Spring (which is also the Goddess' birthday), walk across the bridge to Vesta's farm and Roller will appear.
There are 3 card games you can play in the Casino. Before you play the games you need to find the Sprites that run them! The memory game Sprite, Jum, can be easily found in Romana's fountain. Tep, the Blackjack Sprite, is hidden in the potbelly stove next to the Inn, and Hops can be found if you check out the well next to the Blue Bar.
European Players: The Sprite Casino is different in your version of the game. All 3 Casino Sprites are available, but you can only play the Memory matching game.
The games reward you with Medal currency. You can win Medals from the Casino games and then exchange them for prizes, or use them to hire the Sprite Teams to work on your farm. The Medals are only used within the Sprite Company Tree but you can see how many Medals you have by tapping the stylus on your portrait from the Farm Summary menu. You can exchange 10 G for 1 Medal by talking to Roller, but you can't exchange Medals back into G (although you can sell some of the prizes to Van for profit.)
The most Medals you can bet in a game is 10. If you win you get the opportunity to double your winnings by playing a High-Low game or a High-Select game. If you loose during the double-up then you forfeit all of your winnings for that hand.
High-Low double up
You will be given 2 cards; one is face up while the other is face down. You have to guess if the face down card has a HIGH number, a LOW number, or the SAME number as the face up card. The face down card is then flipped over, and if you win it becomes the face up card for the next round.
In the girl version of HMDS the SAME card option has been removed. If the two cards have a matching value when the face down is flipped over, you get to play again with a new set of cards and you don't loose any Medals.
High Select double up
In this version you are given 5 cards. One of the cards is face up, and the others are all face down. To win the round you have to pick the face down card that is a higher value than the face up card. You have no way of knowing which one of the 4 cards is larger, but it could only be 1 card or even all 4 cards.
When you win one a double-up you can choose to double-up again, or stop and return to the betting screen. Then you just repeat the process until you have the amount of Medals you want.