What kind of calendar? Is it a calendar you manually constructed, or is it a
script-generated calendar?
If it is a manually constructed calendar, simply make the text of a function
into an image and call-up the image in an HTML fragment:
<img src="foldername/imagefilename" alt="enter additional text here">
When the mouse runs over the function's image, a text box will pop-up and
display the additional text. The pop-up is a little unstable and often does
not hold for long. There is another method but it's a bit more complicated to
explain.
