Select the month, copy, paste special as a table. You will have to re-size the
table to fit the space, but now you can bold the dates.
Another solution would be to create individual months through the Design
Gallery. Each month would have to be re-sized, but the calendar is already in
table form.
If you have Publisher 2003 the Design Gallery has a wizard wand on the bottom of
the calendar month. Once you re-size the calendar month, you can copy/paste and
change the dates with the wizard. You only have to re-size once.