For December 15th, if you go to the Google home page in a number of countries and languages, you’ll be greeted by the Esperanto flag! Link to Google France.
December 15th is considered the anniversary of the publication of the language by Zamenhof, and is the closest thing to a ‘national’ holiday for Esperanto speakers.
Weirdly, if you go to Google Canada, you only get the flag if you select French. And if you go to Google Poland, you only get the flag if you select Polish. After looking around a bit more, it seems to be only non-English pages. And countries that have only English pages, such as Australia, don’t have the flag at all.