Pack layers for travel: New York can be cold while Mexico City is cool to mild in December, so bring a light coat, comfortable shoes, and a compact umbrella. Acclimate to Mexico City's altitude by staying hydrated and avoiding strenuous activity on arrival. Purchase travel insurance to cover winter-weather cancellations, and keep digital copies of itineraries and passports. For holiday travel, arrive early at John F. Kennedy International Airport (JFK), monitor flight status often, and consider refundable or flexible fares when booking December flights to Mexico City International Airport (MEX). CTA: Prepare for December travel—review packing lists and buy travel protection before your JFK to MEX flight.