org-log-done

org-log-done is a variable defined in `org.el'.
Its value is

nil

Documentation:
Information to record when a task moves to the DONE state.

Possible values are:

nil Don't add anything, just change the keyword
time Add a time stamp to the task
note Prompt for a note and add it with template `org-log-note-headings'

This option can also be set with on a per-file-basis with

#+STARTUP: nologdone
#+STARTUP: logdone
#+STARTUP: lognotedone

You can have local logging settings for a subtree by setting the LOGGING
property to one or more of these keywords.

You can customize this variable.