Calendar is a personal organiser program that was distributed with Microsoft Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 from version 1.01

Windows 1.0

Windows 1.0 is a 16-bit graphical operating environment, developed by Microsoft and released on 20 November 1985. It was Microsoft's first attempt to implement a multi-tasking graphical user interface-based operating environment on the PC platform. Windows 1.0 was the first version of Windows...

 until Windows 3.11

Windows 3.1x

Windows 3.1x is a series of 16-bit operating systems produced by Microsoft for use on personal computers. The series began with Windows 3.1, which was first sold during March 1992 as a successor to Windows 3.0...

. It was superseded by the calendar in Microsoft Schedule+, which was included in Windows for Workgroups

Windows 3.1x

Windows 3.1x is a series of 16-bit operating systems produced by Microsoft for use on personal computers. The series began with Windows 3.1, which was first sold during March 1992 as a successor to Windows 3.0...

 and Windows NT

Windows NT

Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...

. Windows 95

Windows 95

Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...

, Windows 2000

Windows 2000

Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...

 and their descendants did not include a calendar program until Windows Vista

Windows Vista

Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...

, but Windows XP

Windows XP

Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...

 can run Microsoft Calendar.
Note: Some Windows Longhorn betas have a clock app on the sidebar, that when opened, a calendar applet will open.

Saved Calendar files have the extension .cal. The Calendar file format was one of those described in Volume 4 of Microsoft's Windows 3.1 Programmer's Reference.

Two 32-bit versions of Microsoft Calendar were found in the Windows NT/2000 source trees: one was a simple port of Calendar from 16 to 32-bit, and the other was a Unicode-compatible version. However, the applet did not make the final cut, along with implementations of Microsoft Cardfile

Cardfile

Cardfile is a personal information manager that was distributed with Microsoft Windows starting from the original version 1.01 until Windows NT 4.0 Server. Cardfile is also included with Windows 98 and Windows Millennium Edition, but has to be installed manually from the installation CD-ROM....

, Clock, Cruel Solitaire

Cruel (solitaire)

Cruel is a solitaire card game played using a deck of 52 playing cards. Its arrangement and game play are almost like another solitaire card game Perseverance....

, Golf Solitaire, Pegged (peg solitaire), Reversi, Snake, and Tic-Tac-Toe.