Use a calendar as desktop wallpaper

1. Install dates

2. Open Dates

3. In Calendar > Properties > create new

   Provide the link to ical (.ics) calendar; remove https://

4. install compizconfig-settings-manager

5. Open Startup Applications

6. Add
   

7. Open CompizConfig Settings Manager

8. Go to Window Management -> Window Rules

   

9. In Size rules (adjust to your monitor)

10. Go to Window Management -> Place Windows -> Fixed Window Placement

11. Under Windows with fixed positions (you might want to set Y = 24(task-bar height), to show the menu bar)

    

Thank you Kory Wnuk for the inspiration your answer gave me.

using conky and gcalcli

One way is to use conky to do that task. Here's a tutorial for it.

Basically it says to use gcalcli to connect to your Google calendar, and use the .conkyrc script there to display it. you can change fonts and colors in that file too.

There's a more detailed Ubuntu forums thread about it.

You could use Rainlendar

The normal version is free; the version including network synced calendars costs ~10€. I got to say, though, those 10€ a couple years ago rank among my best value-for-money purchases ever.