The played history is auto cleared when Tracktor is closed. In Windows each history playlist is saved in: "My Documents\Native Instruments\Traktor\History", or in your own root directory if you have set it to a different location (for instance I use "F:\_Music-Traktor\Root Dir\History\")
The file date and time is the time stamp when playign finished (so will be the next day if you played past midnight).
Opening A History Playlist File
You can open one of these playlists by right clicking Playlists and selecting 'Import Playlist'.
Delete the playlist to close it (the history file itself isn't deleted).