The Wikipedia answer:
César Chávez Day
Cesar Chavez Day is a federal commemorative holiday in the U.S. by proclamation of President Obama in 2014. On March 31 of each year, it celebrates the birth and legacy of the civil rights and labor movement activist Cesar Chavez.
For much greater depth:
Friday is César Chávez Day, but who was César Chávez? by Marie Albiges (Community Impact Newspaper)
The Spanish phrase Sí, se puede — “Yes, it can be done” — was used as a rallying chant long before it was echoed in the streets of today’s protests on civil, worker and immigrant rights. It was coined by a man named César Chávez during a 1972 fast in which the Latino American farm worker rights advocate protested a signed Arizona bill that denied farm workers the right to strike and boycott during harvest seasons.
Chávez’ legacy as a leader among farm workers’ unions is honored Friday on what would have been his 90th birthday.