So my initial research into your queries does not really provide a definitive answer to Question 2, but in respect to Question 1, the State Workers Union Day holiday celebrates the Labourer/Technician caste in particular (imo) and the virtues of honest labour and unity in line with the ideology that the Caldari State and it’s constituent Megacorporations preaches for it’s citizens. It’s also a time where the strength of the Union is displayed to rally support for the State, emphasised with military parades and such. I would personally consider this to be an analogy of sorts to the International Labour Day holiday IRL, but fitting for the setting of the Caldari State. To quote the SKIN description below:
“The highlight of the calendar for many Caldari workers is the Caldari Executive Panel mandated State Workers Union Day, one of the few universally observed public holidays in the State. Promotion of the virtues of honestly earned leisure time is exemplified in a special nanocoating that celebrates Union Day, with the proceeds going to the State Workers Union Orphans Apprenticeship Fund.”
It also seems to tie in with Economic Summits and Paramilitary competitions as per this article here, in particular the annual ‘Haadoken Summit and Security Training Games’ as well as an ‘Industrial Planning Summit’.
Secession Day, when the Caldari Chief Executive Panel issued the Proclamation of Secession on November 22nd 23154AD/FC33/BYC82, is the holiday that commemorates the departure of the Caldari State from the Gallente Federation. I am not sure if this has ever been officially commemorated as a CCP/Player event but this would be two months after the Workers Union Day holiday.
I would not wish to guess with certainty how long the State Workers Union Day celebrations have been going on for, but I would assume that it was instituted in the latter years of the Interstellar War with the Gallente Federation or after the Iyen-Oursta Treaty that brought peace to both nation-states. CCP may be able to clarify that for you.