If you’re past level 1000, you’ll have three chances in December to trade Heart Gems for Mouse Tokens. 

Event windows:
• December 8–12 
• December 15–19 
• December 22–26 

During each window, you can make a trade three times. You will receive 500 Mouse Tokens for 20,000 Hearts. 

Watch for these limited-time events in-game and get ready to boost your Token stash!