How to set Monday as the first day of the week in GNOME Calendar applet?

Here's another solution, also from Ubuntu Forums. I think this is somewhat cleaner and more robust: it doesn't involve customizing Ubuntu's locale files (only settings that you're supposed to edit).

Gnome calendar applet adheres to your locale settings. In Ubuntu, you can assign locale components by editing the file /etc/default/locale. Here is what I've got there:

LANG="en_US.UTF-8"
LC_TIME="en_GB.UTF-8"
LC_PAPER="en_GB.UTF-8"
LC_MEASUREMENT="en_GB.UTF-8"

Which means: I want software messages in American, but time, paper size and units in British i.e.weeks starting with Mondays, A4, metric.

Props to artm who posted that!

Of course, LC_TIME="en_GB.UTF-8" is the relevant setting to get weeks to start on Mondays.

You need to log out and back in for the change to take effect.

The best solution for me is to do this on a per-user basis in my own home directory. That way I don't have to edit a system-wide file. (Of course if you want this setting for all of the users on your system you are obviously forced to edit system files.)

What I do is edit the file ~/.xsessionrc to contain the line "export LC_TIME=en_GB.utf8". That's it.

On Ubuntu 12.04 (and later versions I suppose), the easiest way seems to be: System Settings > Language Support, then click on Regional Formats tab and choose from the drop-down list.
In my case I chose "English (Ireland)" which gave me Monday as first day in the calendar, "correct" number formats and currency, and other goodies :)
If you don't find the right option there, you might need to install a language (the other tab of the same window).