Need help with how to match datetime stored in MySQL with a date in a JS calendar

Hi, I’m quite new to coding, and I’m working on a project for school which encompasses using a calendar, and adding event support to it.

So far, I was able to create a popup, as seen in index.php, with text input fields which then sends the input data to insertEvent.php, which inserts it into a database.

However, I can’t seem to find a way to pull the datetime from the database and then match it to a date in the JS calendar itself. I don’t fully understand how the calendar itself works.
Once the calendar and database dates are matched however, it shouldn’t be very hard to create a popup in html to visually indicate where the event is placed.

Any ideas on how to get started on this problem?

Here is the code which is relevant to what I’m trying to do;