Introduction
The calendar combines the functions of a desktop calendar and an
  appointment book to help you keep track of your activities and help
  plan your time.  It provides many of the functions commonly found in
  "Day Planners."
It provides private and shared calendars,
  multi-calendar view, collaboration and scheduling features,
  recurring events (with exceptions and various deletion
  options), alarms, remote iCalendar support, importing/exporting
  of iCalendar and Palm DataBook (PDB) data, an iCalendar
  invitation and view system integrated with IMP, and integration
  with other Horde groupware applications.