Seasonal cases will show up in your game after reaching some milestones in your law firm. They are time-limited challenges that will provide your business with permanent boosters and exclusive rewards. You can find in this very same FAQ section more info about how to play them! 
If you still didn't get them, NO WORRIES! All players will have the opportunity of playing the special cases. They will definitely show up when your law firm is ready.
Please take in mind that there are seasonal in-app purchases available during the event days. Read all the details carefully before you decide to buy a special offer for the season. Among other things, you will be able to unlock all the prizes that appear with a lock in the prize list. However, you will be able to win some permanent boosters and ALL the clues without spending real money.