I would like to display the week numbers in the calendar that is shown in Ubuntu with GNOME shell v3.26 when clicking on the date in bar at the top of the screen.
If you have dconf Editor, launch it and navigate to
/org/gnome/desktop/calendar/show-weekdate. Then disable Use default value and set Custom value to True.
Alternatively just open Terminal and run
gsettings set org.gnome.desktop.calendar show-weekdate true