Open Settings on your iPhone
Tap your name at the top
Tap iCloud
Turn on Calendar
Open the Calendar app
Tap Calendars at the bottom
Make sure the calendars you want are selected
If using Google Calendar, go to Settings
Tap Apps or Mail, Contacts, Calendars
Tap Calendar Accounts
Select your account or add it
Turn on Calendar sync
If using Outlook, add your Outlook account in Settings
Turn on Calendar for that account
Open the Calendar app and refresh it
Check that your iPhone is connected to Wi-Fi or cellular data
Make sure Date and Time are set correctly
Restart your iPhone if the calendar does not sync
Update iOS if needed