Three days and three nights / on the third day
Here’s what it means
Ancient inclusive counting can treat part of a day as belonging to that numbered day.
Why the Bible says it this way
This helps explain passages where 'after three days,' 'on the third day,' and a three-day period describe the same span, though individual texts still require careful study.
The writer uses vivid or unusual wording to make the point clear and memorable.
A helpful detail: The relation of these formulas is frequently discussed and should not be reduced to a modern stopwatch calculation.