Once humans go into space to stay, the Gregorian calendar isn't something they'll necessarily take with them. A timekeeping system based around equinox and solstice and organized for the needs of an agricultural society may not have much more than nostalgic value to the pioneers of Sol, considering that the environments they inhabit will be purely artificial. Depending on the tempo with which space is settled, humanity's new worlds may well ditch Earth's dating system in favor of one which is relevant to them, and not just an apron string binding them across the light-hours to a land that's no longer home.
Ceres is one potential middle-term colonization prospect, given sufficient impetus and sufficiently low launch costs. With a perihelion of 2.54 AUs and a required delta-v of 9.4 km/s from Earth orbit to Ceres orbit, it would probably languish in the shadow of Mars, which is somewhat closer to Earth and has an orbit-to-orbit delta-v of only 5.7 km/s - Hohmann orbits, though. The issues of relative inconvenience go out the window if one has a ship capable of accelerating and decelerating all the way.
Once NASA's Dawn probe arrives there in 2015, a great deal of questions we've currently got on the books about Ceres may well be answered. For one - the question of whether it has underground water ice. That alone would be absolutely vital to any kind of colonization enterprise. Setting down landers without a source of water isn't a colony, or even a caravanserai - it's a dependency of the most base and coarse nature, and I doubt there would be many people who'd pay to have water lobbed across the solar system when those enterprising colonists could have set out instead for Europa or Enceladus. Water is life, remember.
Presuming that Ceres does have water ice, it's sounding to me like the sort of place a fringe colony - presuming it could find enough investors or venture capitalists to pony up the start-up funds - might set up. And once they got there, hey... why not make a new calendar, relevant to their new circumstances, to celebrate their new life?
Ceres' orbital period is a little over four and a half Earth years - 1680.5 days, and the Cererean calendar divides this out into 1,680 twenty-four hour days with a bit left over at the end. The days are grouped into twenty-one months, each eighty days long, and owing to my vision of Ceres as being run along technocratic lines, they are for the most part named after scientists, astronomers, and people relevant to the discovery of Ceres. Here they are:
Dawn, Piazzi, Hirayama, Maskelyne, Dyson, Wolf, Chandrasekhar, Al-Battani, Von Zach, Kirkwood, Messier, Cassini, Hayabusa, Galileo, Herschel, Biederman, Bode, Olbers, Gauss, and Vespers.
One interesting historical synchronicity I found while researching this calendar originally was that Giuseppe Piazzi's first observation of Ceres came on January 1, 1801, the first day of the nineteenth century. This serves as the calendar's zero date, and corresponds to Dawn 1, 0 AC (Anno Cereris). My original notes, which have since been lost in the shuffle, included a formula I kludged together to translate a Gregorian calendar date to a Cererean one, but with this shared starting point it's roughly possible to figure something out.
Very roughly, though, because there's still the issue of that .5 of a day left over. Originally I could have just chopped it off and decreed that, for the purposes of timekeeping, Ceres' year was precisely 1,680 twenty-four hour periods in length. While that would work from an individual perspective, as someone living in Ceres might not care much where exactly the sun was in the sky, this would have left the calendar prone to drifting. I can't see any sense in building a calendar from the ground up and intentionally leaving it unbalanced.
To that end, to fill in the extra time, Ceres has an extra "day" which isn't a day at all. Where the Gregorian calendar jumps smoothly from December 31 to January 1, the Cererean calendar does not similarly transition from Vespers 80 to Dawn 1. Instead, the space between them - 19 hours, 39 minutes, and 21 seconds by my original calculations, although I may have been off - constitutes Year's End. I can imagine this is would be the day when Cerereans drink to excess; think of it as a temporal Vegas, when anything that happens stays there.
Writing out this post, and coming to the slow realization that there are probably a great deal of mathematical shortcomings in it that would blow my brilliant concept to flinders, has made me want to drink to excess myself. Excelsior!