Your probably using Internet Explorer, because on it the calendar looks okay-- IE gets it wrong, though! That's important to remember.
Firefox and Opera render the calendar overlapping the the container above it-- they render your CSS correctly. Rule of thumb: style for the Gecko browsers (Netscape, Mozilla, Firefox). Hack to IE!
top: to 125px corrected it for me. However, to be sure add 1px dashed fuschia borders around the containers. That will make it easier to de-bug. Then check cross-browser, all screen resolutions, try text-zoom on Firefox and Opera.
The "Body for Life" ad, overlaps the right column. I didn't really check carefully, but setting the table to align-left, rather than center, may(?) help this.
The font-size on the right column is kind of tiny at 800 x 600. I would think at higher screen resolutons it will verge on unreadable.
On index.php/archives/ the *long* URI "new cheese" breaks the container. A fast & dirty fix may be to use a URL cut service (Google for a free one) to shorten the URI. If that doesn't work you'll need to do some style work...
Once you're really, really comfortable with styling you may want to accomadate IE for text zooming. Your fonts are set in pixels, and IE can't zoom pixels-- only the Gecko's and Opera can do that. You would have to use % or a combination of %/em's-- these make all browsers happy. But that can wait...