Calendar or contacts not syncing between devices
Usually your devices are using different accounts. Once they all use the same one, they agree.
What's usually going on
A phone can hold several calendars at once: iCloud, Google, Outlook, and one that lives only on the phone. If an event is saved to a calendar the other device doesn't use, it never appears there. Contacts work the same way.
Worth trying first
Find where it was saved.
Open the event on the device where it shows. It will say which calendar it belongs to, such as iCloud, Gmail, or On My iPhone.
Check the default calendar.
In your phone's Settings, open Calendar and look at Default Calendar. Set it to the account your other devices use, so new events go there.
Turn the calendar on everywhere.
On each device, check that the same account is signed in and that Calendars and Contacts are switched on for it.
Look on the web.
Sign in at icloud.com, calendar.google.com, or outlook.com. If an event is there, the account is fine and one device needs to catch up.
Restart and wait a few minutes.
Syncing isn't instant. A restart often prompts a stubborn device to check again.
When to hand it over
If the household shares calendars across Apple, Google, and Microsoft, an assistant needs to see your calendar, or contacts have been duplicated many times over. We'll pick one home for everything and make every device agree.
Your own technologist, on call.
Tell us a little about your setup. We'll take it from there.