An interesting new feature of iOS 9 is auto email event suggestions for your Calendar. The software scans your emails to see if there are any events you haven't added to your schedule. This can be very helpful, especially if a lot of your emails contain pertinent events. However, if you find yourself with a cluttered calendar, you can easily turn off the automatic addition.
Open Settings and tap on Mail, Contacts, Calendars.
Scroll to the bottom: you'll see Events Found in Mail as the last option. Slide the circle left, and you're done.
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