Last Thursdayism, also Last Tuesdayism and Last Wednesdayism, is an atheistic parody which suggests that the universe was created last Thursday with the apparent age of an ancient universe. It is a bizarre parody because no one disputes that contemporary laws of physics could not have been true at the moment of creation, or soon thereafter, rendering aging of the universe based on contemporary laws of physics problematic.
The fallacy in this parody is that it fails to recognize that the creation of something out of nothing will inevitably appear to the naive to be older than it actually is. A creation of a man, for example, would appear to a naive observer as though he existed for decades.
Purpose of the Parody
The parody attempts to use ridicule to conceal logical flaws in assertions of an old universe and Earth. The parody seeks to intimidate anyone into fearing ridicule if he points out fallacies in claims of an old age of the universe and Earth.
Specifically, this atheistic parody, similar to its Invisible Pink Unicorn, targets arguments that the universe is about 6,000 years old. Claims of an older age are based on unproven assumptions of invariance of decay rates and contemporary laws of physics.
The parody especially targets claims that the universe "appears" ancient, but is "actually" much younger. This is not a claim advanced by creation scientists, and so Last Thursdayism is a straw man parody.