Mars' Calendar

Time on Mars is easily divided into days based on its rotation rate and years based on its orbit. Sols, or Martian solar days, are only 39 minutes and 35 seconds longer than Earth days, and there are 668 sols (684 Earth days) in a Martian year.
For convenience, sols are divided into a 24-hour clock. Each landed Mars mission keeps track of "Local Solar Time," or LST, at its landing site, because Local Solar Time relates directly to the position of the Sun in the sky and thus the angle from which camera views are illuminated. The time of day, Local Solar Time, depends upon the lander's longitude on Mars.
Unlike on Earth, there is no leisurely-orbiting moon to give Mars "months," and while there have been many imaginative calendars suggested for Mars, none is in common use. The way that scientists mark the time of Mars year is to use 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.
On Earth, spring, summer, autumn, and winter are all similar in length, because Earth's orbit is nearly circular, so it moves at nearly constant speed around the Sun. By contrast, Mars' elliptical orbit makes its distance from the Sun change with time, and also makes it speed up and slow down in its orbit. 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, and 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°.
The coincidence of aphelion with northern summer solstice means that the climate in the northern hemisphere is more temperate than in the southern hemisphere.  In the south, summers are hot and quick, winters long and cold.
Ls marks the passage of time within a Mars year.  To count up the passage of time from one Mars year to the next, Mars scientists have settled upon the following convention:

For the purpose of this comparison, we use the solar longitude range 0°-360° to define a Mars year and adopt April 11, 1955 (Ls=0°) as the beginning of year 1. In this arbitrary convention, the Mariner 9, Viking, Phobos, and Pathfinder missions occurred in years 9-10, 12-15, 19-20, and 23, respectively. By comparison, the 1992-1999 [Earth-based] millimeter observations extend over years 21-24, and the 1997-1999 [Mars Global Surveyor] TES observations extend over years 23 and 24.

Tables of Seasonal Data for Mars

Martian years and start dates of northern hemisphere seasons
Mars year	Spring equinox (Ls = 0°)	Summer solstice (Ls = 90°)	Autumnal equinox (Ls = 180°)	Winter solstice (Ls = 270°)
01	Apr 11 1955	Oct 27 1955	Apr 27 1956	Sep 21 1956
02	Feb 26 1957	Sep 13 1957	Mar 15 1958	Aug 09 1958
03	Jan 14 1959	Aug 01 1959	Jan 31 1960	Jun 26 1960
04	Dec 01 1960	Jun 18 1961	Dec 18 1961	May 14 1962
05	Oct 19 1962	May 05 1963	Nov 05 1963	Mar 31 1964
06	Sep 05 1964	Mar 22 1965	Sep 22 1965	Feb 15 1966
07	Jul 24 1966	Feb 07 1967	Aug 10 1967	Jan 03 1968
08	Jun 10 1968	Dec 25 1968	Jun 27 1969	Nov 20 1969
09	Apr 28 1970	Nov 12 1970	May 15 1971	Oct 08 1971
10	Mar 15 1972	Sep 29 1972	Apr 01 1973	Aug 25 1973
11	Jan 31 1974	Aug 17 1974	Feb 17 1975	Jul 13 1975
12	Dec 19 1975	Jul 04 1976	Jan 04 1977	May 30 1977
13	Nov 05 1977	May 22 1978	Nov 22 1978	Apr 17 1979
14	Sep 23 1979	Apr 08 1980	Oct 09 1980	Mar 04 1981
15	Aug 10 1981	Feb 24 1982	Aug 27 1982	Jan 20 1983
16	Jun 28 1983	Jan 12 1984	Jul 14 1984	Dec 07 1984
17	May 15 1985	Nov 29 1985	Jun Jan 1986	Oct 25 1986
18	Apr 01 1987	Oct 17 1987	Apr 18 1988	Sep 11 1988
19	Feb 16 1989	Sep 03 1989	Mar 06 1990	Jul 30 1990
20	Jan 04 1991	Jul 22 1991	Jan 22 1992	Jun 16 1992
21	Nov 21 1992	Jun 08 1993	Dec 08 1993	May 04 1994
22	Oct 09 1994	Apr 26 1995	Oct 26 1995	Mar 21 1996
23	Aug 26 1996	Mar 13 1997	Sep 12 1997	Feb 06 1998
24	Jul 14 1998	Jan 29 1999	Jul 31 1999	Dec 25 1999
25	May 31 2000	Dec 16 2000	Jun 17 2001	Nov 11 2001
26	Apr 18 2002	Nov 03 2002	May 05 2003	Sep 29 2003
27	Mar 05 2004	Sep 20 2004	Mar 22 2005	Aug 16 2005
28	Jan 21 2006	Aug 08 2006	Feb 07 2007	Jul 04 2007
29	Dec 09 2007	Jun 25 2008	Dec 25 2008	May 21 2009
30	Oct 26 2009	May 13 2010	Nov 12 2010	Apr 08 2011
31	Sep 13 2011	Mar 30 2012	Sep 29 2012	Feb 23 2013
32	Jul 31 2013	Feb 15 2014	Aug 17 2014	Jan 11 2015
33	Jun 18 2015	Jan 03 2016	Jul 04 2016	Nov 28 2016
34	May 05 2017	Nov 20 2017	May 22 2018	Oct 16 2018
35	Mar 23 2019	Oct 08 2019	Apr 08 2020	Sep 02 2020
36	Feb 07 2021	Aug 25 2021	Feb 24 2022	Jul 21 2022
37	Dec 26 2022	Jul 12 2023	Jan 12 2024	Jun 07 2024
38	Nov 12 2024	May 29 2025	Nov 29 2025	Apr 25 2026
39	Sep 30 2026	Apr 16 2027	Oct 17 2027	Mar 12 2028
40	Aug 17 2028	Mar 03 2029	Sep 03 2029	Jan 28 2030

This table is taken from a 2010 paper by Bruce Cantor, Philip James, and Wendy Calvin: "MARCI and MOC observations of the atmosphere and surface cap in the north polar region of Mars," employing a convention described originally in a 2000 paper by Todd Clancy and several coauthors: "An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere."
Here are how some major events in Mars exploration shake out, according to this calendar:

Martian Years and Seasons for Significant Mission Events
Earth Date	Mars year/Ls	Event
July 14, 1965	6.143	Mariner 4 flyby
August 1969	8.200	Mariner 6 and 7 flybys
November 1971	9.284	Mariner 9, Mars 2, and Mars 3 enter orbit
August 1972	10.64	Mars 2 and 3 shut down
October 27, 1972	10.102	Mariner 9 shuts down
February 1974	11.0	Mars 4 and 5 enter orbit
July 1976	12.88	Viking 1 Orbiter & Lander arrive
September 1976	12.116	Viking 2 Orbiter & Lander arrive
July 25, 1978	13.118	Viking 2 Orbiter shuts down
April 11, 1980	14.91	Viking 2 Lander shuts down
August 17, 1980	14.151	Viking 1 Orbiter shuts down
November 13, 1982	15.226	Viking 1 Lander shuts down
January 29, 1989	19.350	Phobos 2 enters orbit
March 27, 1989	20.18	Phobos 2 shuts down
July 4, 1997	23.142	Mars Pathfinder lands
September 1997	23.173	Mars Global Surveyor enters orbit; Mars Pathfinder shuts down
October 24, 2001	25.258	Mars Odyssey enters orbit
December 14, 2003	26.315	Nozomi flies past Mars
January 2004	26.325	Mars Express, Spirit, and Opportunity arrive
March 10, 2006	28.22	Mars Reconnaisance Orbiter arrives
November 5, 2006	28.130	Mars Global Surveyor shuts down
May 25, 2008	29.76	Phoenix lands
November 2, 2008	29.151	Phoenix shuts down
March 22, 2010	30.67	Last contact with Spirit
August 6, 2012	31.150	Curiosity lands