Turn your computer's desktop into a functional calendar with the accurately named Desktop Calendar.
Once installed, this program blends into the desktop background of your PC. All the icons already present remain accessible. Double click in the appropriate box to make a note, appointment, or to do list on that day. Common holidays are built in and can be turned on with the Festivals option.
That said, this is a very basic program. There is no way to hide the calendar and the individual squares cannot be customized beyond typing messages. It's a great fit for users used to paper calendars on their wall or desk, but not for user who want extra features.
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