Lightning

Lightning is an add-on for Thunderbird, the Mozilla email client, that integrates the Sunbird calendar utility into its interface.

The application lets you create individual calendars and access calendars shared by various users online. The supported calendar network formats are iCalendar (ICS), CalDAV, and Sun Java System Calendar Server (WCAP).

Functionally speaking, there is no difference between Sunbird and any of these other options. Use Lightning to plan tasks or add events you have to attend. Then set up reminders so you don't forget about anything. You can also display tasks and events by day, week, month, etc.