Open the Settings app on your iPhone
Tap Apps
Tap Calendar
Tap Calendar Accounts
Tap Add Account
Choose your calendar service
Sign in to your account
Turn on Calendar sync
Repeat for any additional calendar accounts
Open the Calendar app
Tap Calendars at the bottom
Select the calendars you want to display
Make sure your calendars are set to sync in each account’s settings
Check that your iPhone is connected to the internet
Refresh the Calendar app if events do not appear
Restart your iPhone if syncing still does not work