Yuzu is a golfball-sized citrus fruit that originated in East Asia. It's very tart, with little pulp and lots of seeds. It's very fragrant, and the juice is used to make candies, and the fragrance is captured and put in bath salts. Yuzu is also an essential ingredient in ponzu sauce, and the rind is used to make a tea. We wrote a post on using yuzu in cocktails with shiso leaves.
Fresu yuzu fruit are hard to find in the US. I've seen them a few times at Nijiya Supermarket in San Francisco and Mitsuwa Supermarket in Los Angeles, but only at certain times of the year. You can buy bottled yuzu juice online, or you can grow your own yuzu tree.
In Japan during the winter solstice, many public hot baths and hot springs throw yuzu fruit in the water. Doing so adds a citrus aroma to the water. The Japanese consider this good luck; it's believed that bathing in yuzu on the winter solstice will ward off illness in the coming year. And that's what I'll be doing on Monday; going to a sento (public hot bath) in Tokyo to bathe in yuzu.
If you can't find yuzu, feel free to substitute your favorite citrus, and relax in your bathtub. Welcome the official day of winter with the sunny, fresh aroma of citrus floating with you in a hot bath.