Mentioning silly season provoked me to look up where it comes from. As you may guess from its current circulation — the term is better known in Commonwealth countries than the US — it was a British invention.
The Oxford English Dictionary cites it appearing first in the Saturday Review of London on 13 July 1861. I can find no earlier example.
The Morning Chronicle referred to the term four days later, specifically mentioning the Saturday Review; six months later an article in The Sheffield & Rotherham Independent likewise gave it as the source. Others followed. It would seem that it had indeed been created by a writer on that journal. It referred to the months of August and September, when Parliament and the law courts were on vacation and anybody of substance was away. (Today, the dates are variously specified to suit local conditions.) News was sparse and to fill their columns journalists were forced to feature less significant stories that they wouldn’t bother with during the rest of the year. National papers suffered, but local ones starved, as was illustrated three years later:
The “silly season,” as the Saturday Review calls it, is at its height. Enormous gooseberries and marvellous aerolites are in full force in local newspapers, and happy are the sub-editors who can “congratulate their worthy fellow-townsman, Mr. Such-a-One, on the beautiful effect produced in his back yard by the painting of his pump and water-butt.”
The Illustrated Times, Middlesex, 27 Aug. 1864. An aerolite is a stony meteorite.