The meditation work is in way similar to metal work.
If a blacksmith wants to create something from a piece of metal, the blacksmith will heat the metal in fire. Once the metal gets to some temperature, the metal becomes slightly soft, then the blacksmith will repeatedly hit the metal to give it the desired shape.
While working the metal, the temperature gradually comes down, the metal hardens again. The blacksmith puts the metal back into the fire again and heats it up and then start shaping the metal again.
So the temperature should be within a certain range. The metal should not be cold, otherwise the metal is hard and so difficult to shape. It should not be too hot either, otherwise the metal becomes too soft and does not hold the shape or even melts.
Meditation is similar to heating up the metal.
Our minds and nervous systems are normally like hard metal. They are flexible in children but as we grow up as adults, the capacity to learn reduces.The capacity to change reduces. By the age of ~24 the person has a fully developed personality. Changes do occur after this, but these changes are more from maturity perspective and not from an organic perspective.
So for adults, the functional patterns and habits stay stable for years. Their personality is pretty stable too. Hence the adult mind is similar to cold metal.
Meditation, temporarily, creates a little bit of softness in the structure. This is why, after meditation, we feel different. As time goes, we come back to normal, to our regular self, similar to cooling down of the metal.
For blacksmith, how quickly the metal cools down depends on multiple factors. In winter, the metal cools quickly. For some metals, depending on the composition of the alloy, it takes shorter or longer.
Similarly, cooling time after meditation varies as well.
On average this takes 2 whole sleep cycles. Which is 2 full days.
For some people, their personality is too stiff and the system will come to normal pretty quickly, even within a day. For some others, and especially if the meditation was done with intense focus, the cooling down might take longer, sometimes up to a week.
For blacksmith, if the metal is heated too frequently, it might become too soft and then it might lose its shape. Similarly meditation should not be done too frequently, otherwise it is difficult to safely shape the system.
The soft phase, for us, is an opportunity to make changes.
If you do constructive activities during this phase and if you remain positive overall, when the cooling happens, the system will integrate the positivity in to your personality.
If you do negative or unhealthy activities during this phase, then these will integrate in to your personality.
So the actual meditation is important but what you do after meditation is equally important too.
Safe slots are time slots where a person feels psychologically safe and relatively less stressed.
As we saw above, meditation creates an opportunity window of a few days where changes can happen. For doing positive changes, its better that the opportunity opens during safe slots.
Typically, working class people have free weekends, which is Saturday and Sunday. If you are not going to work on these days, you will be less stressed. So this is a safe 2 days slot. Hence meditating on Friday night is a good idea. You can then have comfortable two days and that will give you more persistent positive results.
This is also one of the reasons why meditation retreats are useful. These are typically week long vacations, in natural and pleasant setting and allow the mind to settle better.
Avoid unsafe slots, at least in the beginning. For example, if you have exam results to be declared on a particular day, it is better to not meditate on the day before. Then if the results do not turn out to be good, you will be less affected.
Note that these opportunity windows and soft phases are not just about meditation. Every intense experience, every altered state of mind, is this soft phase. For example, if you go through a painful breakup, you will be in an altered state of mind for quite some time. You will feel vulnerable, touchy and sensitive. Eventually, hopefully, you will return to normal. But you will not be exactly the same person that you were before the breakup. What happens during this recovery period decides how you recover. If you keep thinking that you are not a good person during this phase, you might end up to be a person with lower self-esteem. On the other hand, if you keep thinking that, maybe you are a good person, but the other person was just incompatible, then you would recover to be a more matured person. Note that in this example, there is not just change in your mood but there is change happening to your personality. This is why maturity, safety and support during recovery phases is so important. Similar soft phases occur after intense positive life events as well.