CGIL and UIL agreed to the strike on Wednesday, Ansa reported. Italy's other trade union confederation – CISL – will also stage a strike next month, but will not participate in the action on December 12th, Ansa said, citing unnamed sources.
CGIL originally said the strike would be held on December 5th.
There were violent clashes last Friday as strikes, backed by labour unions Cub, Usi and ADL Cobas, were held in 20 Italian cities.
The strikes followed a demonstration in Rome at the end of October.
The main focus of union anger has been Renzi's plans for a shake-up of Italy's labour market via the so-called Jobs Act currently going through parliament.
The draft legislation is designed to make it easier for companies to hire and fire - a step the government argues will lead to higher employment levels over time
Unions have also been angered by the Renzi government's plans to cut state spending to help ensure that Italy meets European Union budget deficit targets.
The austerity elements of a package that also includes tax cuts for the poor and employers have increased recently as a result of the economy teetering on the brink of a third recession in six years.
The strike was initially tipped to take place on December 5th, but Renzi's allies were quick to point out that the unions had hedged against a low
turnout by proposing to hold it at the start of a long holiday weekend (the Monday, December 8th, is a public holiday).