Sorry, the article is in Spanish, I've not found an English article about this strike.
From the article:
Almost silently and like a living tide, the call for the Strike for Palestine on September 27 , called by the unions CGT and Solidaridad Obrera (SO), has been gaining strength. Having launched the official call towards the end of August, little by little other non-majority unions (COS, SAT Granada, Co.bas...), organizations, groups from various sectors of society have shown their intention to subscribe to it. Dozens of posters in various territories of the Spanish State are already circulating, referring not only to continuing the strike but also to making it a day of active struggle throughout the day.
The General Strike has not the support of the two mayor Spanish unions (CCOO and UGT)
The manifesto, posters, support/joining form for associations and calls for picket lines/demonstrations in Catalonia (the part in Spain) are accessible from this website: https://defensempalestina.org/ (in Catalan)
Tread cautiously here if you're going to strike. I'm not a lawyer so take this just as a heads up, but Spanish law is tricky in this regard so get informed before risking your job. As per the Ministry of Work and Social Economy it is illegal to strike for 'political reasons, solidarity or support' ('Son ilegales las huelgas por motivos políticos, las de solidaridad o apoyo...'), on top of this you can be not permitted to strike for many other reasons. Pretty fascist I know but what we want is fewer people starving and homeless not more.
Edit: Looked it up, this one's legal (at the time of this comment).
Not a lawer and this is not lawer advice, but the trick usually is to add a lawful reasson in the official communication of the strike. It worked, at least, in the ~~90s~~ 2000s with a 1 hour strike against the war in Irak and more recently in the strikes in the strugle for the independence Catalonia.
Edit: oops, dates...