We have Darwin Day, why not Alexander von Humboldt day (September 14)? If you ask for the greatest natural scientist (a term not used until much later) in the 19th century, you would probably get Darwin. How about number two?
In these days of specialization, the range of topics people like von Humboldt examined is remarkable. Geography, volcanology, mineralogy, electromagnetism, biology, meteorology, and oceanography were all impacted by his work. On his famous South American travels, which later influenced Darwin on his journey on the Beagle, von Humboldt proved the link between the Amazon and Orinoco Rivers, found the earth's magnetic field decreased from the poles to the equator, and nearly reached the top of Chimborazo, what was then thought to be the tallest mountain the world.
The image below is a copy of his diagram of Chimborazo. Not only is it a beautiful image, but it also contains loads of information he gathered while climbing the peak. This includes data about altitude, appearance of electricity, vegetation, changes in weight, blueness of the sky, animals, location of the snowline, the boiling point of water, and atmospheric pressure, temperature, humidity, and composition. The site hosting it has even translated some of the German notes in English. So go take a look this von Humboldt day!