Can I have a desktop shortcut for Google Calendar?

You most certainly can ... though the method is not exactly the most intuitive.

Here's how to do it with Firefox:

1. Click on the icon to the immediate left of the URL you want to open. This will probably be a lock as Google Calendar uses SSL:

2. Drag that lock over to the desktop and let go. This will give you an ugly icon:

   

   Note: If the ugly icon is "good enough", there's nothing more you need to do. Double-clicking the icon will open your preferred browser to the page that you have specified.

3. To make the icon less of an eye-sore, right click the item and choose "Properties"

4. Change the name to something you would prefer, and set an image if you have one that you'd like to see:

   

   Note: You'll see the name on the desktop change as you change the name, but the icon will not update to the one you've specified. This is in the next step.

5. Click the "Permissions" tab, and check "Allow executing file as program":

6. Enjoy your desktop shortcut:

That's all there is to it.