Adding a calendar account is pretty straightforward - you must've done that when you first launched Calendars. But did you know you could use multiple calendar accounts at the same time? For example, you might use iCloud calendar for birthdays, Outlook for work stuff, a few Google accounts for personal and family planning.
Here's how you add a new account:
That's it. All of the connected calendars will sit side by side in your preferred view, so you can see if your work meeting overlaps with taking your daughter to the dentist.
PRO tip: On the sidebar, you'll be able to hide specific calendars to not clutter your schedule. Also, you can mute some calendars that you still want to see but don't want to get alerts from. To do that, just tap the icon.