The Historic Trend section provides information starting from the beginning of the previous calendar week or month. If you select Weekly, you'll see the Top 15 Keywords occurring since the beginning of last week with a graph of the keywords' daily occurence. If you select Monthly, you'll see the Top 15 Keywords occurring since the beginning of last month with a graph of the keywords' occurence by week. The Weekly or Monthly Keyword Trend graph also shows the elapsed days in the current week. So the Weekly Keyword Trend graph will show 7 to 13 days. The Monthly Keyword Trend graph will show 4 to 9 weeks and up to 6 days.