Check you inventory for animals/ decorations from the list above. A lot of things have a chance to drop coal.
I finished too. I like the Winter Bonfire animation and I like the Wizard a lot (of course!).
I got a Earthen Beam and Potion from Greta so I at least know 'she's working'.
But with the timer and the tiny drop rate I'm not that enthusiastic about her anymore. (she does look nice)
Good luck to everyone still trying to finish!