A period (typically in late summer and early autumn) when newspapers (and other media) often cover trivial material because of a lack of more important news.
In the United Kingdom, the supposed lack of important news is a result of Parliament's summer recess in August and September, along with similar breaks at other institutions. In the southern hemisphere, the term is applied to the Christmas and New Year holiday period.
Mid 19th century; earliest use found in Saturday Review. From silly + season.