Where were you last Pluterday? is a serio-comic science fiction novel by Paul van Herck, originally released in Dutch in 1968 as Sam, of de Pluterdag by J.M. Meulenhoff (in their science fiction and fantasy range M=SF as the 14th book of the range) and released in English by DAW books in 1973.
The novel won the first prize at the European Science Fiction Convention (EuroCon) in Trieste, Italy.
The theme in the book is Pluterday, an extra day in the week which can be withdrawn if one saves enough time (e.g. by taking a plane instead of a train). Only the rich can save enough time and thus Pluterday is in practice reserved for the "happy few", resulting in a class society. The existence of Pluterdays is kept secret from non-privileged people.