McDonald's Corporation is encouraging its franchises to be open on Christmas without offering any additional overtime pay to workers.

"[O]ur restaurants will be open to serve our customers when and how they need over the holidays," McDonald's spokesperson Heather Oldani told Ad Age in a report published on Monday, adding that "when our company-owned restaurants are open on the holidays, the staff voluntarily sign up to work. There is no regular overtime pay."

In a Nov. 8 memo, McDonald's USA Chief Operating Officer Jim Johannesen pushed franchises to "ensure your restaurants are open throughout the holidays."

"Our largest holiday opportunity as a system is Christmas Day," Johannesen wrote. "Last year, [company-operated] restaurants that opened on Christmas averaged $5,500 in sales."

A second Dec. 12 memo reportedly disclosed $36 million in additional sales -- or about $6,000 per restaurant -- by opening on Thanksgiving.

An Ad Age analysis determined that if all McDonald's restaurants opened on Christmas, it could mean an additional $84 million at a time when the corporation saw sales decline in October for the first time in nine years.

Hedgeye analyst Howard Penney explained to Ad Age that McDonald's sales had become stagnate because it had already gotten the maximum benefit from a move to keep restaurants open later at night.

"They kind of maxed that out -- all the restaurants that made sense to have open longer, they've already got those sales dollars -- and that whole initiative kind of dried up in 2012," Penney said. "These kinds of things have a limited-growth opportunity year over year."