If you've created your own calendars on your iPhone or iPad, there's probably one that you create more events on than the others. Your Calendar app will default to the generic calendar but if it's not the one you want, you can easily change it to default to the one you use most. This makes creating events much quicker since you don't have to toggle calendars when you create each individual event or appointment.
Not sure how? Follow along.
That's all there is to it. Any new events created from now on will default to the calendar you've chosen. The only time you'll need to change it is if you want to create an event outside of that calendar.