Mexico City in December

October through April is peak season for good reason — the weather is damn near perfect. Highs hover around 75°F, humidity stays low, and rain is rare. December and January get chilly at night (bring a jacket), but days are sunny and crisp. May through September is rainy season, which sounds worse than it is. Most rain comes in afternoon thunderstorms that clear within an hour. The city turns green, hotel prices drop 30%, and you'll have major attractions mostly to yourself. Avoid Holy Week (the week before Easter) and Christmas week unless you enjoy crowds and inflated prices. Día de los Muertos in early November is spectacular but book accommodations months ahead. Here's the thing about Mexico City's altitude: it takes 2-3 days to adjust. Drink more water than you think you need, go easy on alcohol the first night, and don't plan intense activities on day one.