If you have issue where all day events like Day Off are not shown in the Google Calendar app but they are shown in Google Calendar webapp, then you can try following things to resolve your issue.
Do refresh from Google Calendar app.
- App -> 3 dots menu at top right corner -> Refresh
Try reinstalling the google calendar app.
- Sometimes google calendar app is system app. So you can try to uninstall the update of the app and update the app from the play store.
Do full google account sync from OS settings menu. You should look for Accounts option in your OS settings menu then select the google account you want to sync.
If you have any additional questions or remarks, please contact firstname.lastname@example.org