All day event in my calendar is not showing up in my MacBook but it shows up in my iPhone

RWSlater,

Next, what we would recommend, is testing this issue in another user account. What we need to do, is log out of your current user. Log back in under another Standard or Administrator account and see if the same issue occurs there. This way we can tell if it is an issue with your user account, or if it is system wide: Set up users, guests, and groups on Mac

"Add a user

1. On your Mac, choose Apple menu  > System Preferences, then click Users & Groups .
2. If the lock at the bottom left is locked , click it to unlock the preference pane.
3. Click the Add button  below the list of users.
4. Click the New Account pop-up menu, then choose a type of user.

• Administrator: An administrator can add and manage other users, install apps, and change settings. The new user you create when you first set up your Mac is an administrator. Your Mac can have multiple administrators. You can create new ones, and convert standard users to administrators. Don’t set up automatic login for an administrator. If you do, someone could simply restart your Mac and gain access with administrator privileges. To keep your Mac secure, don’t share administrator names and passwords.
• Standard: Standard users are set up by an administrator. Standard users can install apps and change their own settings, but can’t add other users or change other users’ settings.
• Sharing Only: Sharing-only users can access shared files remotely, but can’t log in to or change settings on the computer. To give the user permission to access your shared files or screen, you may need to change settings in the File Sharing, Screen Sharing, or Remote Management pane of Sharing preferences. See Set up file sharing and Share the screen of another Mac.

1. For more information about the options for each type of user, click the Help button in the lower-left corner of the dialog.
2. Enter a full name for the new user. An account name is generated automatically. To use a different account name, enter it now—you can’t change it later.
3. Enter a password for the user, then enter it again to verify. Enter a password hint to help the user remember their password.
4. Click Create User.
5. Depending on the type of user you create, you can also do any of the following:

• For an administrator, select “Allow user to administer this computer.”
• For an administrator, select “Allow user to reset a password using Apple ID.”
• Use Sharing preferences to specify whether the user can share your files and share your screen.

For information about Apple’s privacy policy, see the Apple Privacy Policy website.

If your Mac or Magic Keyboard has Touch ID, a new user can add a fingerprint after logging in to the Mac."

Cheers!

Hi RWSlater,

If the event is created on your iPhone, does it appear on your Mac at all? Test this will help isolate this issue to the Mac itself.

Has the Calendar app been closed out completely and your MacBook restarted since noticing this? Closing and app and restating your computer can help resolve many unexpected issues we may face with our devices.

To quit the app you can select Calendar > Quit Calendar

Once closed out you can go to the Apple menu (  ) > Restart.

Shut down or restart your Mac

You can also confirm that your Mac has any available macOS updates installed. Pre-installed apps such as Calendar update when the macOS is updates so installing the latest macOS will ensure you have the most up to date version of the app and any bug fixes that may be available have been installed.

Update macOS on Mac- Apple Support

Hope that helps!