Calendar Gadget is a simple application for your desktop that displays a calendar. The calendar can be moved across the screen, but you cannot customize its size, which is pretty big, nor minimize it to the tray. It allows you to add appointments, selecting a title, category, start date and time, duration, description, and you can even add reminders and recurrence. The calendar offers a simple but nice user interface and you can also change its skin and even create your own using pictures or photos. You can add borders and transparency, but you cannot customize how transparent the window is. Calendar Gadget also allows you to take two types of snapshots of the calendar: normal and small, but I really can't find in what way that may be useful.
In short, Calendar Gadget is just a simple calendar for your desktop, which includes the ability to add appointments with reminders, but stills lacks a few important features. However, it is completely free, so I suggest you try it and see if it works for your needs.