March 14 when written as 3.14 in date notation looks the mathematical constant pi (π), the ratio of a circle's circumference (distance around the edge of a circle) to its diameter (straight line across a circle through its center).
Due to this similarity, the U.S. House of Representatives in 2009 passed a resolution declaring March 14 "National Pi Day". The first celebrations were held in 1988.
Websites with more information:
Celebrate Pi Day: Seven Classroom Resources for Pi Learning (Edutopia)
How America Celebrates Pi Day (CNN)
Pi Day - Celebrate Mathematics on March 14 (Piday.org)
Pi (π) Day March 14, 2020 (Exploratorium)
These are a few of the books and e-books related to mathematics accessible to LWLC patrons (text & selections: NF):