The carrier flew 1.5 million passengers in the month up from 1.3 in 2014.
The total number of seats rose by 21% to 1.8 million with load factor up 0.8 percentage points to 85.1%.
During December the company expanded its fleet to 65 aircraft, taking on a second new A321ceo.
It also announced 11 new routes in December: five in Romania, three in Macedonia, two in Poland and one in Lithuania.