The Mars Mahadasha is the period of Mangala in the Vimshottari dasha, lasting seven years. In the fixed sequence it follows the Moon and precedes Rahu. As the significator of energy, courage, conflict, and drive, Mars in its dasha is traditionally read as a time of action, competition, and assertion, when matters requiring force or initiative come to a head and disputes may sharpen, the results shaped by Mars's strength and placement in the birth chart. The dasha a person is born into is set by the nakshatra of the natal Moon. These are the interpretive conventions of the tradition; the Vimshottari system is a mathematical timing scheme keyed to the Moon's birth nakshatra, dividing the hundred-and-twenty-year cycle among the nine grahas in fixed shares.
No surviving classical text explains why the nine periods carry the particular lengths they do, seven years for Ketu, twenty for Venus, six for the Sun, ten for the Moon, seven for Mars, eighteen for Rahu, sixteen for Jupiter, nineteen for Saturn, and seventeen for Mercury; Brihat Parashara Hora Shastra gives the figures without deriving them, and modern scholars of Jyotisha have proposed various numerological and astronomical rationales without settling on one that is clearly original to the system. What is agreed is that the nine figures were fixed so as to sum to exactly one hundred and twenty, taken as a full symbolic human lifespan, and that this total, not any individual planet's period, is the number that gives Vimshottari, "one hundred and twenty," its name.