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If you want to keep track of when a certain TODO item was finished, turn on logging with1
(setq org-log-done t)
Then each time you turn a TODO entry into DONE using either C-c C-t in the Org-mode buffer or t in the agenda buffer, a line `CLOSED: [timestamp]' will be inserted just after the headline. If you turn the entry back into a TODO item through further state cycling, that line will be removed again. In the timeline (see Timeline) and in the agenda (see Weekly/Daily agenda), you can then use the l key to display the TODO items closed on each day, giving you an overview of what has been done on a day. If you want to record a note along with the timestamp, use2
(setq org-log-done '(done))