What is the ideal temperature for pouring concrete and what is the lowest temperature that you should pour concrete?

There is no set temperature that prohibits you from pouring concrete. However, it should be noted that concrete poured in very cold conditions (say below 32º F), needs to be protected from the elements. Concrete should not be poured on frozen ground, as this ground is typically "heaved" from frost. After placement of concrete in cold conditions, it needs to be properly cured and kept from falling below an internal temp of 40º F. At this temperature, the hydration almost stops and very little strength gain is acheived. This means keeping it warm with artificial means with heaters and blankets until it has gained enough strength.

As far as an ideal temperature for pouring concrete, I look at it on my own comfort level. If it is too cold or too hot, many negative things can happen to the concrete. A good 65º day, with high humidity, and little wind is an excellent environment. (The high humidity and little wind keeps moisture on the surface of the concrete and limits plastic shrinkage cracking.)