language parameters gem calendar_date_select

Put this at the very end of environment.rb (*after* the 'end' of the
Rails::Initializer.run method):

CalendarDateSelect.format = :finnish

Now I remember I had the same problem when trying to use British-
format dates (dd/mm/yyyy). Try this as a starting point:
http://groups.google.com/group/calendar_date_select/browse_thread/thread/86d0451c515c2074/9f93a4a5976119fa
That discussion is getting old now, so it might be worth looking
through the calendar_date_select group to see if there have been
changes to Rails and/or the plugin that you need to know about. It's
still working for me with Rails 2.2, though.

And more info here:
http://blog.nominet.org.uk/tech/2007/06/14/date-and-time-formating-issues-in-ruby-on-rails/