It turns out this is based on science but is also somewhat arbitrary. I know that sounds like an oxymoron, but bear with me. This is fun.
Mars is the fourth planet from the Sun. On average it orbits about 228 million kilometers out, compared to 150 million for Earth. Because of that, it orbits more slowly and has a longer path to cover, so its year is longer than ours: about 687 (Earth) days compared to 365 and change for our planet.
Measuring the length of the Mars year is all fine and good, but when do you
start that measurement? What day do you choose as Day 1?