This solution is suitable for those who have multiple email accounts set up on your iOS device (iPhone, iPad or iPod). This is useful if you typically send most of your emails through one mail account, but this will also effect calender synchronisation.
1. Launch 'Settings' from your home screen.
2. Tap 'Mail, Contacts, Calenders.
3. Scroll down to 'Default Account' option
4. Tap Default Account and choose the account you'd like your phone to use as the primary account.
5. If you would also like to select a default calender, tap 'Settings' the the top left.
6. Then scroll to the bottom and tap 'Default Calendar'.
7. Like the 'Default Email Account', select the calendar you wise to be your default.