Change OS X date and time format in menu bar

There are 3rd party apps as Day 0. Check if it does what you want and if it is compatible with your version of OS X.

Also you can set modifications from Terminal using a command like:

defaults write com.apple.menuextra.clock "DateFormat" "EEE MMM d  h:mm:ss"

You have to substitute the string between the quotes. As this is not enough documented you have to tinker with it.

The change used to take effect immediately but in recent versions of OS X the defaults are in a "prefrence cache" and you can't see the changes immediately. In that case you need to issue a killall SystemUIServer to reload the Clock.menu Menu Extra preferences.

Nothing guarantee that these changes are applied permanently so it's possible that they revert to the original format after a reboot.

Use http://nsdateformatter.com/ as a reference for the date/time format modifiers, but keep in mind the menubar accepts only a subset of them. To check if the modifier is supported, read out the date format back after it was applied (the delay is needed to the UI server to reload):

$ defaults write com.apple.menuextra.clock "DateFormat" "d MMM w HH:mm" \
  && killall SystemUIServer \
  && sleep 2 && defaults read com.apple.menuextra.clock "DateFormat"
d MMM  HH:mm

As you see, the week modifier is not supported and stripped from the format string. This post suggests to modify the Full date format to include what you need (week number in my case) and it will show up when you click on the date/time entry in the menubar: