If you are new to Calendars
Unless you have already upgraded when you first launched Calendars, you'll be offered to start with the Calendars subscription every time you try using the Pro features (i.e. the functionality that comes as a part of the subscription). You'll be upgraded once you review the subscription terms and confirm the action in the Apple's pop-up message.
The payment will be charged to your Apple ID. The subscription renews automatically until you cancel it. Learn more about our Terms of Service and Privacy Policy.
If you are a Calendars user
If you are already subscribed to Calendars PRO for your iPhone or iPad, you automatically get access to Calendars for Mac. You only need to be logged in with the same Apple ID on both devices.