When the Card Tournament starts, you will immediately be prompted to select your Lucky Card. Each time you collect your chosen card on the solitaire levels, you will receive additional tournament points.

You may change your Lucky Card an unlimited number of times throughout the tournament. To change your Lucky Card, tap on the tournament icon in the main menu and then on the question mark in the upper left corner of the window. On the bottom right, you will see a Lucky Card button, and by tapping it, you will be offered a whole deck of cards to choose from.