While it may increase total output, I doubt it will increase productivity (output per hour worked). The Greek government needs more income. They can increase the tax rate (bad idea), improve tax collection (apparently too difficult culturally), or increase the total number of taxed transactions ... the measure makes sense, at least temporarily, but the devil, as always, lies in the implementation details...
RE: Greece's 6-Day Work Week: A Bold Move & Its Potential Economic Impact