Open source display a desktop calendar : Sunbird from Mozilla

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.
Lightning is a popular calendaring, scheduling and task management extension. Mozilla Sunbird® is a cross-platform application that brings Mozilla-style ease-of-use to your calendar.
Sunbird is available for :

• Windows, English
• Linux x86, English
• Linux x86-64, English
• Mac OS X, English
• Other Languages

Sunbird System Requirements :

Windows

Operating Systems

• Windows XP (Recommended)
• Windows Server 2003
• Windows Vista
• Windows 7
• Windows 2000¹

Minimum Hardware

• Pentium 233 MHz (Recommended: Pentium III 500MHz or greater)
• Windows 7, Windows Vista, Windows XP: 786 MB RAM (Recommended: 1GB RAM or greater)
• 10 MB hard drive space

Mac OS X

Operating Systems

Mac OS X 10.4.x and later

Minimum Hardware

• Macintosh computer with an Intel x86 or PowerPC G3, G4, or G5 processor
• 256 MB RAM (Recommended: 512 MB RAM or greater)
• 10 MB hard drive space

Linux

Software Requirements

Please note that Linux distributors may provide packages for your distribution which have different requirements.

• Linux kernel - 2.2.14 or higher with the following libraries or packages:
  • glibc 2.3.2 or higher
  • XFree86-3.3.6 or higher
  • gtk+2.0 or higher
  • fontconfig (also known as xft)

Minimum Hardware

• Intel Pentium II or AMD K6-III+ 233 MHz CPU (Recommended: 500MHz or greater)
• 64 MB RAM (Recommended: 128 MB RAM or greater)
• 10 MB hard drive space

Learn more about Sunbird and free download.