The basic time periods from which the calendar is constructed are the Martian solar day (sometimes called a sol) and the Martian tropical year. The sol is 39 minutes longer that the Terrestrial solar day and the Martian tropical year is approximately 668.6 sols in length. The basic intercalation formula therefore allocates six 669-sol years and four 668-sol years to each Martian decade. The former (still called leap years even though they are more common that non-leap years) are years that are either odd (not evenly divisible by 2) or else are evenly divisible by 10. The year is divided into 24 months. The first 5 months in each quarter have 28 sols. The final month has only 27 sols unless it is the final month of a leap year when it contains the leap sol as its final sol.
The calendar maintains a seven sol week, but the week is restarted from its first sol at the start of each month (ie the final sol of the week is omitted at the end of each 27-sol month). This is partly for tidiness, but can be rationalised as making the average length of the Martian week close the average length of the Terrestrial week.
The Martian year is treated as beginning near the equinox marking spring in the northern hemisphere of the planet. Mars currently has an axial inclination similar to that of the Earth, so the Martian seasons are perceptible, though the greater eccentricity of Mars' orbit about the Sun compared with that of the Earth means that their significance is strongly amplified in one hemisphere and masked in the other. The most sophisticated calculations of the Darian calendar extend to the point of making allowance for Martian precession. These prescribe a more complicated intercalation formula (for details see the link cited below).
Certain details of the Darian calendar have been the subject of dispute. The names of the 24 months (provisionally chosen as the names of constellations of the zodiac and their Sanskrit equivalents in alternation) and the 7 sols of the week (provisionally named after the Sun, Phobos and the 5 brightest planets as seen from Mars - including the Earth) were less contentious than the selection of the Martian epoch. Originally this was chosen as late 1975 in recognition of the American Viking program as the first fully successful soft landing mission to Mars. This was recognised as being excessively parochial and the currently favoured epoch is early in the 17th century in recognition of Johannes Kepler's use of Tycho Brahe's observations of Mars to elucidate the laws of planetary motion.