i thought it was any coal you hadnt used (getting 7 for a gift pile prize or trading them) by 2nd of january goes into some sort of prize draw for every game on steam. Quote from steam news below.
"Coal is the ticket to enter the Epic Holiday Giveaway. It's called Epic because the top prize is every game on Steam. The more coal you have, the better your chance of winning. Players may alternatively choose to magically mix their coal into something shiny and new by “crafting†seven pieces into a gift from the Great Gift Pile. So this year receiving coal is a good thing."
each piece of coal you had left is effectively one raffle ticket for the epic giveaway, i think the prize draw is at the end of january.