The birth chart in the Horoscope is a table of planetary situation at the time of the birth of Native. Astrologer prefer this table for future prediction called vimsottari Dasha.Mahadasha has Antar dasha (sub period) and this antar dasha again comprises Pratyantar dasha (sub sub period) of planet.
Whenever Mahadasha (Period of Planet) of any planet start, some get married at the beginning of the Mahadasha ( due to transit changes , get benefited by benefic/marak planets), some get the job or get promotion in the job or any abroad placement. child birth in family may happens due to these transit changes. Or some time we experience unfavourable time due to the maha dasha of malefic planet.
A period of 120 years is taken as a mahadasha , period of each planet as : Sun 6 years, Moon 10 Years, Mars 7 Years, Rahu 18 Years, Jupiter 16 Years, Saturn 19 Years, Mercury 17 Years, Ketu 27 Years and Venus 20 Years. These planets rule over these years. This period of 120 years is called the Major Period or also known as Maha dasa life cycle.
In the Mahadasha of any planet whether it is malefic planet or benefic the whole period will not completely good or bad. But the Native has to face some up and down when the Antardasha of Malefic planet comes in the Mahadasha of malefic planet.