Set the Default Mail and Calendar on iOS Devices

To check/change the default calendar

1.) On the main screen choose Settings

2.) Then choose Calendar

3.) Scroll down until you see an option labeled Default Calendar.

4.) Once you selected Default Calendar make sure the calendar under the appropriate email account is checked.
 

To check/change the default mail account

1.) On the main screen choose Settings

2.) Then choose Mail

3.) Scroll down until you see an option labeled Default Account.

4.) Once you selected Default Account make sure the account desired account is checked.