ubuntu application to show calendar with week numbers
Solution 1:
The default time & calendar indicator applet has this built it - you just need to enable the option.
Solution 2:
Running gsettings set org.gnome.shell.calendar show-weekdate true solved the problem for me.
On Ubunu 20.04, the command
gsettings set org.gnome.desktop.calendar show-weekdate true
enables week numbers in the calendar widget.
Solution 3:
On 12.04 after moving from unity to gnome-shell, I had to
gsettings set com.canonical.indicator.datetime show-week-numbers true
to have weeknumbers in the calendar notification applet.