Fait mathématique: 2517 is the number of regions the complex plane is cut into by drawing lines between all pairs of 17^{th} roots of unity.
Culture générale: 2517 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: November 21st is the day in 1918 that Flag of Estonia, previously used by pro-independence activists, is formally adopted as national flag of the Republic of Estonia.