The law also includes New Year's Day, but MSPs agreed not to enable it unless there was evidence staff were being forced to work on January 1.
Shopworkers' union Usdaw welcomed the ban on Christmas Day trading, but added that if stores opened on New Year's Day it would ruin workers' traditional celebrations.
"The Scottish Executive has given us a guarantee that if evidence is produced that retail staff are being forced to work on New Year's Day they will introduce legislation," said general secretary John Hannett. "Usdaw members will be on hand to report employers who coerce their employees to work."
Usdaw will run a campaign in the run-up to Christmas to encourage shopworkers to come forward if they are pressurised by bosses.
However, the Scottish Retail Consortium said it would have been wrong to deny consumers access to shops. "There isn't huge demand for shopping on New Year's Day, but that could change," said director Fiona Moriarty. "There is no reason retailers should be forced to keep the shutters down."