language parameters gem calendar_date_select

Put this at the very end of environment.rb (*after* the 'end' of the 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:
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: