Fasting is advised during a change of seasons, as our bodies acclimatise to the changed weather. One guess is that connecting it to a celebration made it easier for people to follow and change. Heck, smart move. In India, Navratri heralds festivities as the start of winter season and end of monsoon. Biologically, the scriptures believed, at this time our bodies are very vulnerable to the climatic changes and hence need a special routine change in order for it to cope with the imminent changes in weather.
There are primarily 3 types of fasting described in Ayurveda: