Eating Esrog jelly:[1]

Some are accustomed to make Esrog jelly from the Esrog of the Daled Minim. The jelly is consumed on the night of the 15th of Shevat which is the Rosh Hashanah for trees. The consumption of this Esrog jelly is a Segula for women to have an easy birth. [Some[2] are accustomed to also save the Esrog jelly to be eaten on Shavuos.]

[1] Alef Hamagen 660:7; Yifei Laleiv 2 664:15; Likkutei Maharich; See Kesubos 61a; Midrash Raba Vayikra 37b

The Chabad custom: Some individuals received a directive from the Rebbe to use the Esrog to make jelly. [See Otzer Minhagei Chabad 233 that this instruction was given several times]

[2] Mishmeres Shalom 44