While most users have more than one calendar synced to their iPhone, a huge annoyance is having appointments not end up in the correct calendar. Most of the time the default calendar will be the one that is first added to the iPhone. That may not be the most used account.
Sometimes an appointment for a business meeting ends up being synced to a personal calendar, rather than the business account. This could cause the user to forget the engagement.
Luckily this preference can easily be changed in settings.
How to Set Default Calendar.
Scroll down and select Mail, Contacts, Calendars.
Scroll down to the Calendars section and tap Default Calendar.
For most users they should make one of their iCloud accounts, or a Gmail account that they use often, as the default. The biggest thing about choosing the default is realizing that this will be where most of the content ends up. So be sure that is an easily accessible calendar.
After this is changed, all of the newly created appointments in the calendar will be placed in that specific calendar if another calendar is not chosen.
Another reason to change the default calendar is due to Siri. When an appointment is made with Siri the default calendar is where the event is stored.
Some users may only have one calendar linked to their phone, but there are pros and cons to this. On the pro side, there is less confusion about which event goes where and also less data to keep straight. The downside of this is running a single calendar can be too difficult when there are a lot of appointments. If the user only uses the calendar for business or home it may make sense. But with both combined, keeping events straight can become an organizational nightmare.