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:
9 Amazing Pie Charts in Honor of National Pi Day (Washington Post)
Pi Day - Celebrate Mathematics on March 14 (Piday.org)
These are a few of the books and e-books related to mathematics accessible to LWLC patrons (text & selections: NF):