On a world where accessing resources will be challenging in the extreme, and the margin between life and death will be razor thin, an intimate knowledge of Mars’ seasonal cycles will be prerequisite to the success of the colonies. Therefore, a calendar based on the orbital period of Mars is manifestly the best choice for organizing human activity in harmony with the rhythms of its new environment.
On Earth using one revolution about the sun produces something that also tracks the cycle of seasons and therefore conforms to the cycle of life -- of importance to agrarian societies (and nonagrarian societies as well that are dependent upon a biosphere). But basing the Martian calendar upon a revolution about the sun may be a mistake from the standpoint of creating a civil calendar. Even the cycle of life on Mars may be more a function of the timers on the lights in the greenhouse than the external weather. So the basis for the year may best rest upon something else.
Consider that a Martian kid on his 8th birthday will probably be pretty pissed to know that an Earth kid has already had twice as many birthday parties. It might not be too bad to only have to buy your spouse an anniversary present once a Martian year, but maybe nuptials should be celebrated a little more frequently? Or consider the accounting scandals among Martian corporations that only have to file annual reports once every 669 sols. How long will it be before a Martian government decides you have to file you income taxes twice a year as doing it only once a year does not provide an adequate revenue stream to provide the services demanded of a rapidly growing population or to manage a global terraforming project? Who wants a 6 month summer rerun season on TV? Maybe the problem is not that you need to create months to make the Martian year more manageable, maybe the year is just defined too long. The revyear and the civyear could be very different.