Once, [Before the holiday of Sukkot], R. Refael of Bershiv* was tasked by R. Pinchas of Koretz to purchase an etrog (one of the four species taken on Sukkot). Despite the difficulties in obtaining one, R. Refael was ultimately successful in procuring a single etrog for the entire town. Upon his return, R. Refael encountered a Jewish individual who was selling fish, which was also scarce, and R. Refael started to plead with him to sell the fish to Rabbi Pinchas. However, the individual only consented to the sale in exchange for the procured etrog. R. Refael ultimately agreed to the exchange and traded the fish for the etrog. 

When he reached Koretz without the etrog, he was questioned, "How could this happen? What did you do, trading an etrog for a fish?" The first day of that specific Sukkot occurred on a Sabbath. Hence, the practice of taking an etrog was prohibited. – R. Refael explained, “Having fish for Yom Tov (a religious festival) is a sure way to have a joyous celebration, and being joyful during Yom Tov is, after all, a biblical requirement ("And you shall be joyful in your festival"). In contrast, taking an etrog on the second day of Sukkot is purely rabbinical. Therefore, having fish is preferable.” 

Tolner Rebbe

Y.F.

*He was a student of R. Pinchas Koritzer.