A calendar is the perfect reason to use a table! Calendars inherently present tabular data and HTML tables are good at presenting tabular data. And HTML table markup provides nearly all the CSS hooks you need to associate CSS selectors with various parts of the table to dress it up.
I'm all for using DIVs for layout--but stick with tables for tabular data.
Here is a cool article on how to dress up tables with CSS: http://www.smashingmagazine.com/2008/08/13/top-10-css-table-designs