lightening calendar in thunderbird
http://www.mozilla.org/projects/calendar/lightning/
Lightning brings the Sunbird calendar to the popular email client, Mozilla Thunderbird. Since it's an extension, Lightning is tightly integrated with Thunderbird, allowing it to easily perform email-related calendaring tasks.
or sunbird stand alone
http://www.mozilla.org/projects/calendar/sunbird/
Mozilla Sunbird® is a cross-platform calendar application, built upon Mozilla Toolkit. Our goal is to provide you with a full-featured and easy to use calendar application that you can use around the world.