So there are many factors that contribute to this answer but the straight forward answer is this:
considering that the influenza type would either be A or B (only two strains that infect humans) usually occurs during the late fall to early spring season in the Northern Hemisphere (although you can get the flu not matter what time of year this is mostly the highest peak). Infection is spread readily through small airborne droplets through talking, breathing, and coughing. Low humidity and cooling temperatures allow stabilization of the virus with close proximity of individuals during the colder months allows increased spreading. if the virus contaminates a formite (surface) can remain infectious for roughly as long as a day.