Q. Is it correct that attending a Pidyon Haben is equivalent 84 days of fasting? Why?

A. Sdei Chemed mentions that their may not even be a source for the above, besides that it equals one of the longest series of days of fasting (Shovevim). However, others maintain that it is based on the pasuk (Bamidbar 3: 49): And Moshe received the moneys of their ‘Pidyom’ and that word stands for PD – YOM or translated numerically as 84 Days. (Ilana D’Chaye, Mishmeres Shalom 19: 2 and others quoted in Otzar Pidyon Haben 21:3).

The principle of the 84 fasts is also suggested in Ya’akov Avinu brocho to Reuben (Bereshis 49: 3), in which he said, “Reuben, you are my firstborn; my strength, and the first of my body issue.”

As our Sages explain it means that Reuben was conceived from the first seed that left his father’s body, since he did not experience a seminal emission until then. As this most remarkable act occurred when Yaakov was 84 years old, the 84 fasts of the Pidyon of the first born represent the remarkable self-control that Ya’akov Avinu maintained for 84 years (ibid.)

Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a