I don't know how much the seasons would matter to a Martian colonist, but if you're on a planet lit by a day/night cycle that's approximately 24 hours, then you're most likely going to synchronize your sleep schedule to that, not the day/night cycle of some other planet you're not actually living on. That would be your day. And then from there the days and months get out of phase with Earth so you'd end up using your own calendar as well, at least for accounting days of the month. Using Earth months and years might be fine, though, as long as one accepts that the months are a few days shorter and so on; that seems reasonable. And "years" being a very specific chronological length are so well ingrained that you'd almost need to invent a new word for Mars years.