We often are asked why the thirteenth moon from the previous year's Lunaria is sometimes repeated as the first moon in the new year's calendar. The Lunaria is our attempt to combine 13 lunation of 29.53 days (a synodic month) with the traditional solar Gregorian calendar, and it is not a tidy fit. 13 lunation come out to 384 days and so would run over 19 days into the next year. We've found repeating a month is a better alternative than doing a 12 moon calendar every other year or so or getting ahead of the traditional year. The 13th lunation fits in only every 2.5 years, so that a 5 year cycle contains 62 lunation.
We think it is important to start the "month" with the new moon as it is a good time for new beginnings and a natural time to refresh. We also feel it is important to honor the seasonal cycle of the Earth, the equinox and solstices and the astrological correspondences - and so try to include the entire traditional year in each calendar and begin each Lunaria as close to Jan 1st as we can and still get in the winter solstice at the end of the year. This often makes for a repeat for previous readers, but helps those purchasing a new year calendar to start the new year as they are expecting.
Wikipedia has a terrific page on this and the difficulties of lunisolar calendars at http://en.wikipedia.org/wiki/Month#Synodic_month
and this page specific to lunisolar