El 5 de mayo

A) La batalla de Puebla

You cannot get from Veracruz to Chapultepec without going through Puebla, and Mexican forces led by Ignacio Zaragoza defeated the advancing French occupation army taking them en passant on May 5, 1862. France ended up recapturing Puebla one year later, but "el 5 de mayo" bought significant time for Benito Juárez to consolidate and strategically retreat to Northern Mexico where he continued to remotely run presidential operations. It also forced a significant re-ante by Napoleón III in his efforts to install a monarchy with Maximiliano and Carlota at the throne.

Lincoln was a stalwart friend of Mexico and strongly opposed the French occupation, much as he opposed the occupation of Mexico by another thirsty imperial nation 2 decades prior that led to the loss of half its national territory (we also hold Antonio López de Santa Anna accountable for this), but Lincoln now had his hands tied with the civil war. This was initially convenient for Napoleon III as the USA was not able to apply its Monroe Doctrine as a deterrent to France, but the battle of Puebla provided a morale boost to Union forces who were concerned that an early victory by France in Mexico could lead to an ally for the Confederacy. As Mexican forces continued their resistance and as the Union emerged victorious in 1865, Napoleón III had no choice but to eventually withdraw French troops.

Carlota returned to Europe, first to the Miramar Castle in Trieste and then to Tervuren and Bouchout in Belgium where she lived until 1927. Maximiliano remained in Mexico, was captured in Querétaro and executed by firing squad in 1867. Benito Juárez successfully restored the Republic of Mexico.

B) Worldwide Mexican Heritage and Cultural Appreciation Day!

C) Mexican Independence Day

D) A and B but not C.

El cinco de mayo me monto a caballo, me tumba el caballo y me monto en un gallo, me tumba el gallo y me monto en el cinco de mayo.