When everything in the universe was created. Also, the time when many various things happened.
I started my period last Thursday.
Represents a measure of time, being the long-term memory of popular culture (and sixteen year old girls in particular), since both seem to have trouble recalling anything that happened before last Thursday.
As a span of time, Last Thursday lies somewhere between a hundred years ago and the present, and is always followed by last Friday.
It is widely accepted that anything that did not take place this week took place Last Thursday. That is unless this event took place back in the day, which is of course a Wednesday.