Milestone Events occur every Monday and provide you with a themed ladder of rewards and a fun goal: Collect themed Milestone Event Tokens by playing game levels. Themes change regularly, and can appear for up to 8 weeks. The event appears in the upper left corner of the main screen: 

Every time you reach a milestone, you will receive a reward that you can claim immediately, or you may wait and collect all, or some, of your rewards any time before the event ends. You can track your progress by the Milestone Event Meter. As you progress with the event, you’ll unlock more rewards that you can immediately claim.

Once per event, players may choose to acquire Premium Rewards, which allows them to earn both free and premium rewards for each reward step for the duration of that event. This also rewards the player with 2x the Milestone Event Tokens on each level for the remainder of the event. Players who purchase Premium Rewards after claiming some free rewards, will immediately receive the premium reward associated with every free reward already claimed.