Note: If you don't see an answer here that you think should have made the cut, you can nominate it here as well (though do make sure it's up to the standards of the other entries on the shortlist)! Just remember that this thread is for Best of December, not Best of Year (which will come later.)
/u/lord_mayor_of_reddit answered What is the origin of Christmas?
/u/bigglesworth_ answered "How did moving children from cities to the countryside work in WWII Britain?"
/u/alotofreading answered "Why is the Hopi reservation completely surrounded by the Navajo reservation?"
/u/wotan_weevil answered Did a soldiers swordsmanship/skill really matter in large medieval battles?
/u/Total_Markage answered Khan I ask a question about the Mongols?
/u/elgorn answered What was Egyptian life like under Nasser?
/u/shadowsofutopia answered "Was Tuol Sleng the only prison in Cambodia under the Khmer Rouge?"