If you a 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.
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.
If you are a Calendars 5 user
Right now Calendars on Mac is offered only as a part of the Calendars subscription. We might consider some offers for our Calendars 5 customers in the future, also you may subscribe to Calendars and receive all the updates included there.