Actually, various different colleges and high schools hold this event on different dates in early March. On this day, mathematicians encourage teenage girls to consider studying and working in the field of mathematics by holding talks, competitions, and fun activities.
Sonia Kovalevsky, who grew up in Russia and moved to Germany and Sweden as an adult, became in 1874 the first European woman to earn a doctorate in mathematics. She earned the degree summa cum laude, which means with highest honors, even though she was not allowed to take classes, attend lectures, or even sit for examinations. What she did do was to study privately with various teachers and to write and present three scholarly papers in which she offered new contributions to mathematics.
When she first earned her degree, Kovalevsky wasn't allowed to be a lecturer at any university, because she was a woman, even though she offered to provide lectures for free! Finally, starting in 1883, Kovalevsky was able to secure university positions, such as “Professor Without a Chair.” She became one of the first women editors of a scholarly journal, and she finally earned a Professorial Chair at Stockholm University, the first Northern European woman to do so. Even though she died way too young—at age 41—Kovalevsky managed to squeeze having a child and writing two non-academic books in between all her mathematical and women's-rights accomplishments.
Also on this date: