In traditional Hindu Astrology, the Solah Somvar Vrat can prove to be extremely beneficial for those people who are have a malefic Saturn in their horoscopes or those who are going through the turbulent periods of the Shani Sade Sati or Shani Mahadasha.
The Solah Somvar Vrat is also very beneficial in reversing the ill-effect of a malefic Moon in the horoscope. The Moon as is common knowledge is the planet, which effects the mind and is responsible for mood swings and melancholic depression and chronic dis-satisfaction.
Astrology also says that the Solah Somvar Vrat can be most auspicious for those people who are born on a Monday.
The Solah Somvar Vrat is also considered to be beneficial is giving relief to people suffering from lung and eye related diseases and ailments, pregnancy related problems and mental disorders, sickness and ill-health of the mother and lack of help or happiness from children.
The Solah Somvar Vrat can be useful in resolving legal disputes, including court cases and litigation, putting an end to financial loss and all kinds of money problems and hardships.
Traditionally, Indian women are known to perform this Fast, which is dedicated to Lord Shiva for domestic happiness and harmony in their married lives and the long life and prosperity of their husbands. Unmarried women also undertake the Solah Somvar Fast to get a suitable bridegroom with the blessing of Shiva.
The specific rituals related to the Solah Somvar Vrat are primarily based upon local customs and traditions, which have been followed for ages and hence cannot be generalized. As this fast is mostly undertaken by women, in my opinion there is no need to take a break during the Menstrual Periods, the woman can keep the fast but refrain from performing any of the rituals.