What happens when I tap on the Calendar icon?

You can find this icon by tapping the "3-lines" icon in the top-right corner of the Missions menu. Tapping this icon will bring up the Daily Reward screen.

Here, you can collect various rewards that will help you in the game daily.

The daily reward available to you can be collected at any time, even on another day. This can be done by tapping the "Watch" button just below the reward. This will then show you an advert that you will have to watch to claim the reward.

Once a reward is collected, the time that you will need to wait to collect the following reward will be shown at the bottom of it.