15 December is Zamenhof Day (link) for Esperantists. It is the birthday of Ludwig Lazarus Zamenhof, the inventor of the conlang Esperanto. So, if you see somebody wearing a star today, you can say Happy Zamenhof Day to them in Esperanto (see the subject title of this post).
—Ceci n'est pas un seing.
I see we've posted a lot about Esperanto here on Wordcraft. Even our old linguist friend, Aput, posted about it.