Because of its location on a high plateau and its high elevation (over 7000 feet) Mexico City enjoys mild, pleasant weather nearly all year.  It's winters, while cooler than the summers, are very mild.  The temperature in winter averages in the high 60's to low 70's with the average high peaking in the summer and early fall months in the low to mid 80's. 

October through May is the city's dry season, this period having very little rain, while June through September is considered the "rainy season".  During the summer months it rains on average once a day, though the rain rarely lasts longer than a few hours.

Regardless of the season, remember to always carry a light sweater or jacket. On an average day, Mexico City has cool mornings and late evenings (60's) and warm afternoons (high 70's low 80's).

For weather charts of the monthly average high and low temperatures in Celcius and Farenheit  and monthly rainfall averages in millimeters and inches - as well as the up-to-the moment weather from the weather station at Mexico City's Benito Juarez International Airport see Mexico City Weather  

For an up to date weather forecast of Mexico City check The National Weather Service.