Unlike Out of the Gap I have not spoken to any FerroNats representative so I'll assume he has the better information.
Nevertheless the specific term "dias naturales" is used precisely as a number of consecutive days regardless of whether you are working or not on these days.
In any case for your information we at Aena had our vacations previously calculated in cycles... that is you had a specific amount of workshifts free each year, including both your working and rest days on that shift. So if you had a 6+3 roster each cycle was 9 consecutive days.
After the changes Aena made to our working conditions our vacations are 3 15-day periods each year. I'd suggest you ask that question about vacations to FerroNats providing a detailed example of what is meant and demanding a written answer.