How to highlight one calendar square in Apple Calendar?

Hi norafrompali,

Welcome to the Apple Community.

Seems there is no way to actually highlight a square,

except for the automatic red circle around "today".

You can, however,

choose different colours for the events.

add notes / urls to events

customise alerts, and notifications etc.

Here's the :

Calendar User Guide:

NB: Click on "Table of Contents at the top of the Welcome page that opens from the link, to see detailed articles.

For accurate info, choose the macOS version of your Mac at the top of any articles you browse.

https://support.apple.com/en-au/guide/calendar/welcome/mac

All the best :-)

You're welcome, norafrompali,

Thank you for replying, and letting us know you appreciate our community of Apple users, like yourself, doing what we can..

Cheers :-)