Summerfilth

Summerfilth was a non-existent month used jocularly among Shire-hobbits.

Proverbially, Summerfilth would begin on the day of Friday; there was no actual month in the Shire Calendar that ever started on a Friday (since after the reform of c. Third Age 2670, each weekday always fell on the same date) and the Hobbits would say "on Friday the first of Summerfilth" to refer to something that would never happen.