Depending on where you are located, Mosquito Season could be all year long. In some states that may be true.
For other states the mosquito activity will begin when the temperature reaches a consistent level of 45-50° F. At this level the mosquito eggs begin hatching and the mosquito season begins.
PARA'KITO MAP: The shape of mosquito season by state - Using Average Time.
Related information: For additional information about mosquitoes check the CDC website https://www.cdc.gov/mosquitoes/