Novel where the wealthy could save up time and use it for an "eighth day of the week"

Question

Sometime around the 1980s, I recall reading one of my father's 'mass market paperback' science fiction novels. The book was showing its age even then, so it's likely to have come from the 60s or 70s.

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The basic idea of the story was that rich people had a ways of saving time not available to the 'great unwashed', and the saved time could be saved, in a 'bank'. When sufficient saved time was accumulated, the saver could have an extra day, which was conventionally taken between Saturday and Sunday. When the saver 'took' the day, it seemed to be in a world/universe/'phase-of-existance'/somethingorother which was just like the Real World, except that only the people 'taking the day' were there.

The story centered around someone who somehow managed to convince one of the rich people that he was one of them, for social reasons, and he learned about this time-banking scheme during the course of the story, and somehow made it public.

Answer 1

This is the 1973 novel "Where were you last Pluterday?" by Paul van Herck

The wikipedia description seems to fit quite nicely

The theme in the book is Pluterday, an extra day in the week which can be withdrawn if one saves enough time (e.g. by taking a plane instead of a train).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 to non-privileged people.