Many of the other writers have stated that a fortnight is generally thought of as fourteen nights / fifteen days and, as a general rule, I would agree with them.
Where I live, in northern Spain, the phrase ‘quince dias’, fifteen days, would seem to confirm that thinking. When a Spaniard says ‘quince dias’ he means anything from a week to three weeks; succinct is not a word that the Spanish have a nodding acquaintance with.
I am reminded of a holiday my husband and I took, when we still lived in the United Kingdom. We travelled to Jamaica, via Maine. This was the one and only time that we have stepped on American soil, and a very nice place it seemed to be. But I digress.
At that time, we had two cats, and when we went on holiday, they spent their holiday at a local cattery. They both seemed to enjoy the experience, so we had no qualms about leaving them for a fortnight.
We travelled midweek, returning on the Wednesday, or so we thought, and the following day I went to the cattery to collect them. I had booked them in until the Friday, just in case we were a bit jet-lagged from our trip. When I arrived at the cattery and explained I had come a day early, they looked at me a bit strangely. You were not due in until today, they informed me. I had booked the cats in until Friday, I said. This is Friday, they said. Oops! I had completely lost a day, and at that very moment I should have been behind my desk at work, so I quickly went home and called in (not sick but silly) for having lost a day.
In that case, a fortnight was fifteen nights and sixteen days, mainly due to the time difference. There were many similarities between the Jamaicans and the Spaniards in their attitude towards punctuality, laid-back seems too soft a term to adequately describe how lax it can be at times.
We had a fabulous holiday in Jamaica, I would recommend it to anyone. Lovely weather, fabulous golf courses, very friendly people. We did not visit Kingston, there was a bit of unpleasantness going on there at that time, so better safe than sorry. Be prepared for a very noisy experience; if there is one abiding memory of Jamaica, it is the loudness of the people and their music.
These are my experiences regarding fortnights, hopefully they have been helpful and amusing.