One Date Each Month Displays In Blue

I was testing the plugin and created an event for the 23rd in a category designated with blue. Everything appeared and worked as expected.
I then deleted the event and created another on a different date.
This event also appears and works as expected.
The minor problem I’d like to fix for a customer is that the number 23 of every month is blue in color. It is not linked. When I place the pointer on the date box, the number disappears in the gray shadow of the date box.
Did I miss a step when deleting the original?
Thanks.