On days where temperatures are at extreme highs or lows people use a lot of energy as they turn on heaters or ac units. In addition people have on all the other electrical appliances they usually have on such as computers, lights, televisions and many others. 

The Demand Response program is where electricity companies monitor the grid and make sure that the levels are not reaching capacity if it does then large commercial users are paid to cut back on power. This way there is enough power for everyone and the system isn’t overused so there are no brown outs are black outs.

With programs like demand response there is hope that the price of electricity might be able to go down because less power will need to be provided the less people use it.

http://www.bostonglobe.com/business/2013/07/22/demand-response-takes-pressure-off-power-grid/b5gZ6ENPYjmr4KWq4LZpdN/story.html

http://energy.gov/oe/technology-development/smart-grid/demand-response

http://www.ferc.gov/industries/electric/indus-act/demand-response.asp