December is the harvesting time of Yuzu,  Japanese Citrus fruit.
Because it is a Citrus,  probably it cannot grow in Hokkaido or Tohoku(Northern part in Japan).
We don’t really buy it in supermarket(at least I have never bought), it is a thing you can get from your friends, if you live in Japan
We use this useful fruit for many purpose, seasoning ingredient, making jam, for aroma therapy.
It has very unique aroma.  And quite a few recipe is depending on this.