Hey Sherry,
This is not possible without some customization, unfortunately. While we cannot help with customizations, a general idea here would be to put events that are public holidays into a specific Event Category. Then, you could write custom CSS using this category to change the styles for just these events.
If you need to write custom CSS on your site, definitely check out a [free!] tool like Firebug if you use FireFox, or the Developer Tools for either Safari or Chrome. They have “Inspector” tools that let you zoom right over the element whose CSS you’d need to change, and thus make it very very easy to customize.
Best of luck with your customizations!
George