Date Display: How to hide part of repeat rule?
The recurrence description is defined in date_repeat_rrule_description() in date_repeat.module. 

At this time, there is no way for themes to change it. See this issue: http://drupal.org/node/342665

EDIT

Actually, you may override theme_date_repeat_field(), which calls date_repeat_rrule_description(). You could call your own version of that function to customize the display.

This would leave the contrib modules untouched, but you would be duplicating some effort and have more code to maintain.