Martian calendar

credit: NASA, JPL, Caltech

1 Mars sidereal day is 24 hours, 37 minutes and 22 seconds. This is the rotational rate of Mars around its axis.

1 Mars solar day (sol) is 24 hours, 39 minutes and 35 seconds.

1 Mars year is 669.6 sols (686.98 Earth days).

Mars’ orbit around the sun is more elliptical compared to Earth’s orbit. Ellipticity combined with an axial tilt of 25.2° causes the Martian seasons to have different lengths.

• Mars spring in the northern hemisphere (autumn in the southern): 194 sols.
• Mars autumn in the northern hemisphere (spring in the southern): 142 sols.
• Mars northern winter/southern summer: 154 sols
• Mars northern summer/southern winter: 178 sols.

The measure of ellipticity is known as eccentricity.

• Mars eccentricity = 0.0934 (varies on a cycle of 2.5 million years)
• Earth eccentricity = 0.0167086 (varies on a cycle of 100 thousand years)

Currently, Mars axial tilt angle is 25.2° (measured from the line perpendicular to the ecliptic) (similar to Earth’s axial tilt angle which is currently 23.439281°).

Earth’s tilt angle varies between 22.2 and 24.5 degrees with a periodicity of 41 thousand years (Earth’s axial tilt angle is currently decreasing). The tight range of Earth’s axial tilt angle is due to Moon’s stabilizing effect. There is no such effect in the Mars system. The two Martian moons, Phobos and Deimos are too small to stabilize Mars’ axial tilt, so it can vary much more. The periodicity of Mars axial tilt is 2.5 million years.

The “month” concept is not meaningful for Mars. Scientists mark the time of Mars year using solar longitude, abbreviated Ls (read “ell sub ess”). Ls is 0° at the vernal equinox (beginning of northern spring), 90° at summer solstice, 180° at autumnal equinox, and 270° at winter solstice.

“Solar longitude, commonly abbreviated as Ls, is the ecliptic longitude of the Sun, i.e. the position of the Sun on the celestial sphere along the ecliptic. It is also an effective measure of the position of the Earth (or any other Sun-orbiting body) in its orbit around the Sun, usually taken as zero at the moment of the vernal equinox.” – Wikipedia

• Mars is at aphelion (its greatest distance from the Sun, 249 million kilometers, where it moves most slowly) at Ls = 70°, near the northern summer solstice,
• Mars is at perihelion (least distance from the Sun, 207 million kilometers, where it moves fastest) at Ls = 250°, near the southern summer solstice.
• The Mars dust storm season begins just after perihelion at around Ls = 260°.

(In red) Martian season lengths and time as compared to seasons on Earth (in blue), with marks for the vernal equinox, perihelion, and aphelion

Based on these basics a specific Martian calendar was proposed.

https://www.planetary.org/articles/mars-calendar

Note: In order to gain insight about the difference between the sideral day and the solar day you can read “Difference between sideral day and solar day on Earth“. Same logic applies to Mars.