On the Calendars page, select a calendar you wish enable alarms for.
Click the tab Alarms and adjust the alarm settings as you like.
On the home screen of the device, tap Calendar.
Turn the device to portrait orientation, scroll down to Calendar and tap it.
Look for the calendar you wish to unsubscribe and tap on the icon beside it.
Make sure Event Alerts is ON. Finally, tap Done in the top right corner.