Sign in to the same Apple ID on both iPhone and Mac
On iPhone, go to Settings
Tap your name
Tap iCloud
Turn on Calendar
On Mac, open System Settings
Click your Apple ID
Click iCloud
Turn on Calendar
Open the Calendar app on Mac
Make sure the correct calendar account is enabled
On iPhone, open the Calendar app and confirm events are saved to an iCloud calendar
Connect both devices to the internet
Wait a few minutes for syncing to complete
Refresh the Calendar app on Mac if needed
Check that Date & Time settings are set correctly on both devices