I've been out of the game over a year now, but have collected monthly data for roughly 900 boxes through about 6 years. December is traditionally a better month, but not the best. The problem with the data is trying to not let the "mother lodes" obscure the background averages. I had 1 huge hit in May 2015, making May the "best overall month to coin roll hunt."
Overall, including mother lodes, I averaged 3.1 silver per box. If I choose to ignore each month that did better than double the overall average (only 5 times in 6 years -- Mar'15, Apr'18, May'15, Nov'16, Dec'18), here's how it shakes out average silver/box Jan - Dec:
J-1.4 F-1.7 M-2.0 A-2.1 M-1.6 J-1.7 J-1.7 A-2.3 S1.4 O-2.7 N-2.7 D-2.2
December is "ok" but not a particular standout. It's really about volume -- slowly steadily _continuously_ accumulating over time -- and by doing that you're increasing chances to be lucky to hit some random mother lodes along the way. 40% of my silvers came from those 5 big months. Sometimes it was 1 insane box, sometimes it was spread over a few great boxes.
Keep on hunting!