Monterrey has four seasons, and is not what people would expect from other "beach" tourist destinations around Mexico.  It can have a little bit of all types of weather, sometimes in one day!

Generally, summer (May-Sept) is very hot and humid.  Temperatures can go on up over roughly 100F or 40C, with high humidity.  It's not a time you'd want to be outdoors or without air conditioning much.  Winter in Monterrey actually gets quite cold.  I think it has more to do with the lack of insulation in most buildings, because the cold isn't that unusual (it can get near freezing, and has snowed from time to time--though snow in Monterrey is rare). 

I think the best times to visit Monterrey are during spring (March-May) and fall (October-November), which are gorgeous.   It has those cool evenings, and warm sunny days.  There are several wonderful places to go outside and explore, such as Chipinque, Estanzuela, or La Huasteca Canyon.  This time of year is also perfect to just stroll through the neighborhoods, check out the Macro Plaza or Barrio Antiguo downtown, or just sit at an outdoor cafe and enjoy a coffee.

That being said, you really can enjoy yourself in Monterrey anytime.  Just dress appropriately for the weather and be prepared! 

One thing about rain.. I don't think it's very predictable.  There is a heavier rainy season around August-November, but that doesn't mean it's the only time of year there is heavy rain.  There are days where it just looks like there are a few clouds looming overhead, but sure enough, a thunderstorm breaks out a few hours later.  Generally, Monterrey is dry, but when the rain comes, it definitely makes its presence known.  Just keep in mind that the weather here is unpredictable.