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The Diary Project is a year long art project by British artist, Kirsty Hall.
From the site:

Every day in 2007 I am drawing on an envelope, placing something secret inside and posting it to myself before my midnight deadline. When these envelopes return to me they will be kept unopened until they can be exhibited as a whole artwork. Members of the public will then be able to open the letters and investigate the contents when it is exhibited as a whole artwork in 2008. Once the letters have been opened, the contents will be displayed on the Diary Project blog.

To see more of The Diary Project, visit the blog here. – Link.