Modules for dates & events

Date (http://drupal.org/project/date)
Requires CCK. Recommends jQuery UI for popup calendars and time entry widgets.
Defines CCK date/time fields and widgets. What makes this module essential is that it also provides a Date API submodule that provides superior date & time handling than in Drupal core.(This functionality is part of core in Drupal 7.)
jQuery UI (http://drupal.org/project/jquery_ui)
Requires installation of (non-Drupal specific) jQuery UI library
Provides the (non-Drupal) jQuery UI plug-in to other Drupal modules.
Calendar (http://drupal.org/project/calendar)
Requires Views, CCK, Date.
Enables displaying views containing dates as Calendars.