Secretly, "fortnight" is a code word, a contraction of "fourteen nights". But the latter term is a 3 syllabled mouthful, which, with frequent use, slides into the palatable ease of "fortnight".
"Fourteen nights" may be any given fourteen nights from A to B, from any chosen day of the week, in any season, in any country. "Tomorrow fortnight" is a clumsy, but convenient term of reference. I am sure there is some contraction pending on this one too!
Australians and Brits find "fortnight" a convenient term of communication; a term to indicate an exact due date in the near future OR a generic term of approximation indicating a measure of unwillingness to commit to the immediacy of tomorrow. The length of the fortnight may be subject to some suspension of disbelief. For the Americans, "fortnight" is more a museum curiosity. A "sennight", meaning half a fortnight, has become an artifact. No one uses this term. Far easier to shrink this bi-syllabled jog into the flash of "week". Most of us have forgotten, or are even ignorant, we took this momentous step. After all, time is precious! Don't waste time talking about it.
For the dreamers and those who love word associations, "fortnight" sounds remarkably like some stronghold for night. For me, I think of Fort Knox. I know, "knox" is actually a small rounded hill. But the "k" is silent, and we are left with "nox", the base of "nocturnal", the trappings of night. So, still making wild associations, how long is Fort Knox? Fort Knox is as long as the gold bullion within. Hmmmmmmm More questions! How long is the value of gold? Is it as long as riches? Hmmmm Another topic in the making?
A fortnight is more than a week, less than a month.
A fortnight is a specific time in the future or a convenient diversion from committing to a specific, more immediate time frame.