Rainlendar is a free-to-use desktop calendar that's available for Windows, Linux and Mac OS X. I've been using Rainlendar for a couple of years now it is a very impressive tool. The application literally sits on your desktop, displaying the current month and highlights your important dates - whether they be work meetings, holidays or in my case, the football fixtures! :) You can also set reminders for important events - such as when to pay your credit card bill and also recurring events such as people's birthdays and anniversaries. Reminders can be set to play an alarm or display a message on-screen at the relevant time. All events are colour-coded and categories so you can instantly see what type of event is ahead... green=work meeting, yellow=holiday, etc. These can be edited to the colours of your choice. You can download the free (Lite) version from rainlendar.net, or pay 9.95 euros for the Pro version - which is pretty much the same as the Lite version, but allows you to share you calendars and adds Outlook and Google Calendar support. Both versions are less than 5Mb to download and are easy to install.
A customizable desktop calendar.