Last Thursdayism

Last Thursdayism (alternately Last Tuesdayism or Last Wednesdayism) is the idea that the universe was created last Thursday, but with the physical appearance of being billions of years old. It's also a counter to the creationism theory. Under Last Thursdayism, books, fossils, light already on the way from distant stars, and literally everything (including your memories of the time before last Thursday) were all formed at the time of creation (last Thursday) in a state such that they appear much older.

Last Thursdayism functions both as a philosophical point on how our observations may not match with "reality" and a reductio ad absurdum of the young-Earth creationist idea of the omphalos hypothesis: if the world was created 6,000 years ago with the appearance of being made billions of years ago, what is there to stop us from claiming it was made Last Thursday?

The debate on whether Last Thursdayism is true has raged on ever since the creation of the universe last Thursday.

Falsifiability
The following quote from Fundies Say the Darndest Things explains why people find the Last Thursdayism idea difficult to comprehend:

The flaw easily shows in the above argument: if God placed your memories of events before Last Thursday into your mind, then it would appear as if you remembered something "before" Last Thursday. Because Last Thursdayism can explain anything, it operates as an unfalsifiable hypothesis (much like the omphalos hypothesis itself).

Occam's Razor
One can regard Last Thursdayism (and any similar idea) as "hyper-omphalisms" — extreme forms of omphalist doctrine designed to show the absurdity of it. As such, they constitute a skeptical hypothesis of the very existence of "the past", not just of events that allegedly occured within it.

However, the assumption that all of the universe was specially created is unnecessarily complex (given you have all the evidence for an old universe PLUS the assumption that it was deceitfully created to look old); it thus makes more sense, via Occam's Razor, to assume uniformitarianism and an old universe.

Origins
Bertrand Russell explained in 1921 the problems with proving that the universe was created, intact, at a set point in the past in his "five minute hypothesis":

The recent manifestation of Last Thursdayism occurred on the Usenet group talk.origins in 1992, as a hyperbolic response to omphalism and apocalyptic preaching:

It gradually gained popularity; on August 25, 1996, the "Church of Last Thursday FAQ" (an early version of "The Last Thursday Catechism" ) was posted to talk.origins by Michael Keane. This version is similar to Unicornism or Pastafarianism, and claims that the universe was created Last Thursday by "Queen Maeve the Housecat", who on Next Thursday (Judgment Day) will admit those who were nice to cats to Paradise and damn the unkind, the uncaring, and Creationists to the never-cleaned Eternal Litterbox.

In some circles, especially talk.origins, Last Tuesdayism is taken as a schism of Last Thursdayism and is founded on the belief that the world was created last Tuesday — but, unlike last Thursdayism, this happens every Tuesday.

A new incarnation of Last Thursdayism has popped up, and appears to be a kind of solipsism: