Taken from Wikipedia:
"Yuzu is also known for its characteristically strong aroma, and the oil from its skin is marketed as a fragrance. In Japan, bathing with yuzu on Tōji, the winter solstice, is a custom that dates to at least the early 18th century. Whole yuzu fruits are floated in the hot water of the bath, sometimes enclosed in a cloth bag, releasing their aroma. The fruit may also be cut in half, allowing the citrus juice to mingle with the bathwater. The yuzu bath, known commonly as yuzuyu, but also as yuzuburo, is said to guard against colds, treat the roughness of skin, warm the body, and relax the mind."
Oh man I remember playing the flipping card game back in elementary school. I'm so happy it got mentioned here because none of my US friends knows what I'm talking about when I talked about my childhood