Basics:
Shani (Saturn) takes about 30 years to go through 12 houses or 360 degrees
This means on average it takes about 2 and a half years to travel through one house
'Sade Sati' literally means 7 and a half(7.5)
Time Shani takes to travel through 1 house = 2.5 years
Time Shani takes to travel through 3 houses = 3* Time Shani takes to travel through 1 house
= 3*2.5 years
= 7.5 years or in other words 'Sade-Sati'
Now we know it is Shani's movement through 3 houses.
But which specific houses are these?
12th house from the moon
1st house from the moon or the house in which our moon is present at birth
2nd house from the moon
Why these 3 houses are difficult to transit for Shani and not other houses?
Well, well, it's not just Shani's transit but the transit of all planets from the 12th, 1st, and 2nd house from the moon sign that tests you. Then why don't we dread the transit of other planets through these houses and only Shani? The reason is that Shani is a hard taskmaster, he likes things to be balanced. He gives you lessons for all aspects of life you were negligent towards in the past Shani cycle of 22.5 years(30-7.5). Imagine it as a big exam of 7.5 years. He is not there to punish you, just to evaluate you. If you were a good student and prepared well for this exam in the past 22.5 years or if you were born with good Shani karma from past lives, then chances are you will find the exams relatively easy.
Also, other planets move fairly quickly - for example, let us say you have bad mangal(mars) in your birth horoscope. Every time mangal transits through the 12th, 1st, and 2nd house from your moon you will get some level of stress in life. But then how long does that period last? 3-5 months. It's like pulling off a bandaid from a wound compared to a broken bone that can take months to heal i.e. Shani transit.
Why is Shani SadeSati considered from the moon sign and not Lagna?
That is because most struggles in life are in your mind, and which planet signifies mind? It's our moon.
Which aspects of life do the Shani Sadesati impact?
12th house transit - Lose of mass support, stagnancy in career, you are no longer happening, issues with accumulating wealth, unnecessary expenses, you start realizing who is your true well-wisher and who was there to just take advantage of your success during the 9th, 10th, and 11th transit of Shani. (See article on Shani cycle)
1st house transit - Health issues, ego issues, character assassination, loss of courage, trouble in married life, and partnerships.
2nd house transit - Financial issues, hard to get loans, people who lent you money need their money back, loss of liquidity, property problems, etc.
This is just a general guideline. Every horoscope is different and it will impact you in a unique way designed just for you.
A few factors that one needs to consider -
Birth horoscope Shani placement
A good placement generally means good past Shani karma and fewer issues during Sadesati
Lagna or Ascendant
A Shani-friendly Lagna is better e.g. Tula(Libra), Vrishabh(Taurus), Makar(Capricorn), and Kumbh(Aquarius)
Current Mahadasha
Mahadasha of a favorable planet concurrent to Shani Sadesati can help a bit. Mahadasha or an unfavorable planet can make things worse
Even for people with favorable factors, Shani will test your patience, and progress will be slow to come by.
Is it all gloom and doom if all factors are bad for you?
Clean up your act during the 9th, the 10th, and 11th transit of Shani. Be nice to people, don't be rude, and help people below your social strata and old people, avoid alcohol and drugs at all costs.
Make a list of all things that you 'overdo' in your life e.g. over-work, over-eat, over-play, over-rude, over-nice, lazy, etc. you get the idea. These will cause you issues during Sadesati in some form or other. e.g. Over-eat - health issues; over-work - stress issues; over-rude - people will stop supporting you; over-nice - people will take undue advantage; over-spend - will lose money; lazy at work - layoff/career stagnation/demotion; over-exercise - back issues/bone issues
During the Sadesati period do things that help you calm your mind. Again this is different for different people. e.g. Meditation or Hanuman Chalisa for some hard metal music for others; hiking and being alone in nature for some while being with family and playing with kids for others; self-improvement for some volunteering for others.
Put in the hard work with family, work, friends, partners, etc; but don't expect equal results. Lower your expectations in general. This does not mean being lazy or agnostic; just be prepared for not-so-favorable results.
You might not see the favorable results of your hard work during this time, but you definitely see them once Shani transits to the 3rd house or when mahadasha period/antardasha changes.
For some people, gemstones can provide some relief, but again that needs a careful reading of your chart.
Please feel free to reach-out to me at modvedicastro@gmail.com if you have any questions/feedback about this article.