Seasonality
for House Fly
House flies are a warm-weather pest whose numbers peak in the hotter summer months. Source: https://ipm.ucanr.edu/PMG/PESTNOTES/pn7457.html
They don't disappear in winter: summer adults live only a couple of weeks, but cooler conditions can stretch survival to about three months, and some overwinter outdoors in sheltered spots or in cracks inside buildings. Source: https://extension.psu.edu/house-flies