There are three important things for an event to take place in a native’s life - promise, transit and Dasa. For instance, if your 7th house and the lord are positive and natal Jupiter and Venus are also placed well, your chart has a strong “promise” of marriage but for it to take place, “transit” Jupiter and Saturn should be influencing marriage related houses or lord (2nd, 7th, 11th). Another major aspect for event occurrence is “Dasa”.

In Vedic Astrology, different planets influence us at different points of time in our life. The perceived life span of an individual is counted as 120 years in total. Each planet is assigned certain number of years in one’s life. Our life is divided into Mahadasa of 9 planets and each Mahadasa is then further divided into 9 Antardasas each. The Mahadasas follow in the following sequence - Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn, Mercury. Mahadasa begins from the lord of the Nakshatra where is Moon positioned at the time of birth. The exact duration of the first Mahadasa depends upon the degree of the Nakshatra when you were born.

Ketu Mahadasa runs for 7 years, Venus has 20 years, Sun has 6 years, moon has 10 years, Mars has 7 years, Rahu has 18 years, Jupiter has 16 years, Saturn has 19 years, and Mercury has 17 years.  
Mahadasa is like a calendar of your life’s events. Whether the Mahadasa is auspicious or inauspicious for you depends upon several factors like position of that planet in your birth chart, whether it’s malefic or benefic. This duration of planets gives direction to the events in your life.  

Mahadasa of each planet runs for several years but this doesn’t mean that you get the results associated with a single planet only. The actual results depend upon Antadasa. Mahadasa of each planet is further divided into 9 planets Antardasas following the same sequence of planets.

The events of your life are based on both - the Mahadasa and the Antardasa planets. For example, if you have Rahu Antardasa under Rahu Mahadasa, events would follow as per Rahu’s characteristics and strength in your horoscope. However, if you have Jupiter under Rahu, then the results would be based on the position of both Jupiter as well as Rahu. If the planets are positioned on 2 – 12, 6 – 8 or 5 – 9 axis, repercussions might not be that positive for the native. The lordship of planets is also considered in the process. If both planets are lords of Kendra or Trikone houses, the results would be like that of a Raj Yoga. If one has Kendra or Trikone lordship, and the second planet is lord of 6th, 8th or 12th house, then the results might not be favorable.