You asked how we do these layouts for the desktop. The answer is: the same as a regular layout, just in the pixel size of the display monitor. You can find that on a Mac by going to System Preferences--Displays and see what the resolution of your monitor is, for example 1024 x 768 pixels. Then start a new file in PS or PSE at that pixel size, and 300dpi, and add a background and whatever elements you wish to the layout. Flatten the image and save it as a jpg file, at full 300dpi resolution, and it will be ready for the desktop. Go to System Preferences--Desktop & Screen Saver, and find your file; click on it and it will display on your screen immediately.
As for the calendar, various designers have calendar templates available. Do a search in the store for "calendar" and they should come up. Andrea Gold has a full line of desktop calendar templates available for any month any year.
If you have a PC, the place you'll find the pixel dimensions and add the desktop image will be in settings. Perhaps someone else will tell you exactly where.