A joint call out by the CGT and CNT - not something I ever really expected to see.
Links in Spanish:
http://cnt.es/noticias/29-s-la-lucha-está-en-la-calle-hacia-la-huelga-general
[edit: translation here]
http://www.alasbarricadas.org/forums/viewtopic.php?f=20&t=50786
English translation of the original joint statement by the CGT, CNT and Solidaridad Obrera:
http://www.solfed.org.uk/?q=spain-towards-a-general-strike
CO.BAS and the Coordinadora Sindicalismo de Clase are involved in this as well. The latest statement also mentions the participation of ASSI Zaragoza, the Sindicato Asambleario de Sanidad de Madrid and the general strike working group of 15M in Madrid.