A hot citron bath called Yuzuyu in Japanese is the bath in which citrons are put set afloat.
Japanese people have a custom to take a hot citron bath on the very day of the winter solstice.
In the past, people took the citron bath to purify themselves. Since citrons have strong flavor and the old saying suggested that citrons remove negative energy.
Recently, it is said that you wouldn’t catch a cold if you take a hot citron bath on the day and this is why Japanese people keep this custom to take a hot citron bath. There are some bathhouses that prepare this citron bath as well.
Citrons have an effect to facilitate the flow of blood and alleviate the feeling of cold, so it may be true that you can avoid catching a cold ! Also the fruit skin of citrons contains an ingredient to make the skin beautiful thanks to citric acid and vitamin C.