Pardon me for asking such a niche question. In Michat yehuda by Yehuda Ftaya, chapter 88, there is quote from Chaim Vital (Shaar Hakavanot right before Rosh Hashannah 90b) speaking about fasting for 3 days and 3 nights before Rosh Hashannah, Yom Kippur the 10th of Tevet and the 17th of Tammuz to remove all the punishments to befall one that year. Earlier in the book, He states that fasting for 3 days and nights are equal to forty days of fasting. May one interchange these prescriptions and fast forty for one of the days but 3 days and nights for the other? What about fasting one day and night and giving tzedakah for the rest (Michat yehuda quotes Chased L’Avraham in saying 40 pieces of silver, but can that be interchanged with a cash value? If so what would be the equivalent today) May one also fast one day and give tzadakah for the next day and so on until 20 fasts and 20 coins (or their equivalent) are donated? What other general rules must follow the fasting?
Shalom, Thank you for your question. Unfortunately I have to tell you that I do not know the answer. However my assumption is that anyone who needs to ask – is probably not on the level to do the answer! On a more serious note – Rabbi Kook zt”l writes in his letters, that in our generation we should refrain from fasting (except for the required national fast days during the year). The reason for this is that in previous generations they had an over abundance of spiritual and physical energy that could be dissipated through fasting. Such people needed to “use up” their extra forces so as to make sure they did not end up using them for sin. However, in our days, when there is a lack of spiritual energy in the world, one should not take on themselves any optional fasts. In fact, Rav Kook's student, the famous Rav Harlap zt”l was originally a student of another Rabbi who encouraged fasting. After Rav Harlap found Rav Kook, and wanted to be his close student, Rav Kook told him that he would agree to teach him – on the condition that he refrained from any more optional fasts. I hope this is of some help to you - Blessings.