The etrog is the citron fruit used as a component in the arba minim, the four species. Along with the lulav (palm) branches, these represent the agricultural aspect of Sukkot. Great care is given to selecting a blemish-free etrog and caring for it during Sukkot.
There are a number of uses for the etrog. Insert cloves all over the etrog, covering it completely. Once done, you have a fragrant spice box for Havdalah that will smell wonderful and last a long time. This is a great family activity and can be the impetus for adding the ritual of Havdalah to your observance of Shabbat.
The etrog, like other citrus fruits, can be used to make a flavorful jam or marmalade; another recent trend is to make etrog-infused vodka. Enjoy the fragrant etrog during Sukkot and for many weeks after.
