A non-existent day, one that will never come
"In jokes and plays four centuries ago, Tib often referred to a girl with loose morals, so there was no Saint Tib and therefore no Tib's Eve. To say something would happen on Tib's Eve was to say it would never happen."
— Philip Hiscock
"f you research Tibb’s Eve you will likely find references to Tipsy Eve and alcohol. That was not my experience. I don’t remember it being a ‘party’ event and certainly not a ‘drink until you’re tipsy’ event. "
— Robert Hiscock
Tibb (or Tib) was a character in many 17th-century English plays, representing a woman of loose virtue, usually for comic effect. As a fictional character, "Saint Tibb" was also a non-person, consequently could not have a feast day. By extension, Saint Tibb doesn't have any celebration named after her, and in common parlance, Saint Tibb's Eve could never come, and the phrase was used to describe an indistinct (possibly non-existent time) in the future. Someone with a vague promise to pay a debt or perform a function was often spoken of as being ready to deliver on Saint Tibb's Eve, a day unlikely ever to arrive, rather like the year of the linux desktop.
The phrase itself is rarely (if ever) heard these days. In Newfoundland and Labrador there are those who honour the day on 23rd December.¹ The only other reference I know of is to the 'Discordian Calendar's 29th of February leap day, known as Saint Tibs' Day.
Saint Tibb's in Newfoundland
$ xclip -o | wc -w
246
For Terse Quest 2024, also writeup № 666