Q. On question 3515 above in regard to fasting on Tu Bishvat, could the one keeping a Yohrzait Taanis fast next day?
A. Shulchan Aruch (O. H. 568: 9) rules that if the day of Yohrzait for one's father fell on Shabbos or Rosh Chodesh, the fasting should be done next day. However, Remah adds that the minhag is not to fast at all.
See also Responsa of Rabbenu Gershom Meor Hagola (14) that indeed if a community accepted to keep a series of fasting days and one of the coincided with Tu Bishvat, they should skip fasting that day and add another to the list.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller, Horav Dovid Pam and Horav Aharon Miller Shlit'a.
Important Numbers
