Paper turns brown as the  result of exposure to the sun. The longer the paper enjoys the sunbath, the more it will tan. It is a "documentary" calendar. 

It is a rip-off calender counting days in summer (Jun.-Aug.) by showing sunlight exposure to paper. This little experiment has been executed for the duration of a whole summer season in order to expose the reaction of paper to sunlight. A calendar sheet was attached to a window every day. The longer it has been exposed, the more the paper gets burned.  It means the darker of the date will be shown. So 1 June displays the least tan while 31 Augest gets the most.