Is that the mini-calendar module - you can achieve this using CSS very easily. If you give us the URL we can give you some assistance if you need

Thanks you to introduce , I have answer use CSS to change text color and font weight of holiday to example.

Path file to edit CSS with Phoca Commander : components/com_jevents/views/alternative/assets/css/modstyle.css

.mod_events_td_daywithevents > a.mod_events_daylink:link {

    color: red;

    font-weight: 900;

}



.mod_events_td_daywithevents > a.mod_events_daylink:visited {

    color: red;

    font-weight: 900;

}

Credit : https://www.w3schools.com/cssref/sel_element_gt.asp

Please don't change the core CSS file - you will loose your changes when you upgrade.

You should add this to the JEvents custom css file - there is a menu item to allow you to do this from the backend of JEvents