Where do I find that plugin_calendar.tpl file? I can't see it in the obvious places online. Is it something I will find out if I look at the technical information? I found about the possible strftime parameters. (I find a dropdown list of strftime parameters for the main (English language) blog, but it doesn't offer the common %B %d, %Y or %Y-%m-%d - but I'll find out how to change that).You can change the appearance either through CSS and for some plugins also with Template files. The calendar plugin for example uses the "plugin_calendar.tpl" template file for its output, there you can also change the used date format (using PHPs strftime() parameters).
You can change all other parts of your template by editing the Smarty .tpl / template files, have a look at www.s9y.org for more technical information on that.
TIA
Margaret