On Mars, you'll get extra time in the day to do the things you want to do--like sleep in! Martian days are about half an hour longer than Earth days. But that's nothing compared to the extra time you'll get in a year--687 days versus only 365 days on Earth.
The above is an excerpt from a kid book and can also be found on the website below: https://www.windows2universe.org/kids_space/lifeonmars.html
I wonder why the definite article was used in this case.
You will have extra time in a day on Mars
also sounds ok to me.
Maybe "a day" indicate "a single day on Mars"?
I am so lost here and would be really grateful if someone can help me understand this issue. Thank you