There may another term for this in English, but if so I don't know what it is. It's not "off-shift", because that seems to mean evening and night cover. Anyway, this is what it means. It's from Argentina; I don't know whether it exists elsewhere:

"Mencionan que el Decreto Nº 937-GCBA-2007 define al “franquero” como el personal que por la naturaleza de su prestación cumple una jornada de trabajo normal y habitual en días sábados, domingos, feriados, días no laborales o aquellos días que sean considerados asueto, cumpliendo una jornada de doce (12) horas por cada día de trabajo."
http://sites.google.com/site/enfermeria7deabril/franqueros-g...

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Note added at 1 hr (2011-11-17 23:09:01 GMT)
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This definition is very specific. It explicitly says "jornada de trabajo normal y habitual". So "franqueras" work normal hours, but on "off days". Nor do they replace or provide cover for absent staff, except in the sense that they work the days other staff don't. They are not temps or locums. They are simply the normal daytime staff on weekends and holidays, apparently on a permanent basis. I just can't find an exact equivalent for this.