The Calendar app allows you to track your schedule of events (that you created or that you’ve been invited to), organized by day, week, month, year, or agenda. Tap the Calendar menu icon to switch calendar views or access your Tasks list.
When you first set up your phone, you configured it to use an existing Google Account or you created a new account. The first time you open the Calendar app on your phone, it displays any existing calendar events from your Google Account on the web.