Widget Calendar shows wrong event day

I have a calendar in a sidebar widget, with one event assigned to it so far.
The event does not show up in the calendar, and there’s a black square on a day that I don’t recall assigning with a hyperlink to an internal page that doesn’t exist under ‘pages’.

https://perfectattendance.ca/

How do I get events to show up in the calendar?
How do I remove the Sept.11 black square?

Thank you,