Q. My elderly father has problems fasting, but he usually is able to fast. His doctor told him if he prepares and eats properly the weeks before, he can try even in full day fasts. The question arose on fasting on Tzom Gedalyah that is shorter and easier, he didn’t fast because he wanted to fast Yom Kippur. Did he do the right thing or should he rather have fasted Tzom Gedalyah that came first?
A. On a similar question (1853), we wrote: “Poskim disagree whether it is better to observe the taanis now at hand and fast on Tzom Gedalia, even if that may cause not fasting on Yom Kippur, since that would be considered then a “ones” or being beyond one’s control and exempt of any culpability. (Sdei Chemed – M. Yom Kipurim 1: 10).
However, many Poskim maintain and recommend not to fast on Tzom Gedalia if that would jeopardize the fasting on Yom Kippur. (see Tzitz Eliezer 10: 25 and Piskei Teshuvos 602: 1).
Horav Shlomo Miller’s Shlit’a opinion is to follow in principle this last opinion, however a competent Rov should be consulted to verify the reality of the threatening medical condition.”
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a