The fasting is from dawn till nightfall, every new moon for two to three days
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I solved the problem when the "Ramadan" fasting has to be done.
The Quran refers to the night of power as the first revelation of the Quran, and to fast a few days, two to three days, no intercourse, or eat or drink.
The first revelation was August 10th of 610? Whatever the stand of the moon was from there, a few days.
If the new moon isn't visible (the crescent) then fasting is not necessary and feeding the poor is necessary. Example places with eternal sun. Also, 2x12 (months)=24, 3x12=36, Muhammad may have chosen to fast these days and put them into a full month instead because it would be easier to follow.
There is no harm to do the 24 to 36 day fast all together if you want to, but for autophagy and glycogen store depletion, this few day fast every month is the healthiest for the human body, as well.