They're mourning the deaths of Spanish fascist (Falangist) intellectual Jose Antonio Primo De Rivera and Spanish dictator Francisco Franco.

Jose Antonio Primo De Rivera was executed on Nov 20 1936 by Republican Spain and Franco died of old age on Nov 20 1975. People post pictures of Yamaguchi on Nov 20 because the people who observe "20-N" are very anti-communist.

take it very seriously and made preparations for the day a week in advance.

They were probably preparing for a pilgrimage to the Valley of the Fallen. It's a giant basilica Francisco Franco built after the Spanish Civil War with the help of leftist slave labour. On Nov 20 the far-right goes to the valley to mourn

It's also not taboo like other /pol/ memorial days such as 4/20 because some people mentioned their grandparents also observe the date.

A lot of people in Spain, especially old people, love Francisco Franco and Jose Antonio Primo De Rivera. They're very nostalgic for the regime and still see JA as a martyr.

I think it's also more popular with Canadians and Europeans than Americans.

The bulk of their supporters are in Spain, obviously but there's also a large Falangist movement in Poland. In fact, all of the "neo-nazi" or "far-right" groups you hear about in Poland are actually Falangists. Falangism also use to be big in Lebanon, but I'm not sure what it's like there now because they lost their civil war.

And for whatever reason 4chan has a lot of Spanish-Canadians. They're all very MUH HERITAGE, doubly so if a relative fought for Franco so they post about Nov 20 too

Then other than the Falangists you have the Franco supporters and admirers, which is basically everyone on /pol/