There is something near or in your house that has decayed or is decaying,you need to search high and low for what is causing the odor for the flies to be tracking...good luck
Well if csi is true to it's criminal investigations then the flies are there because something is dead inside your house like a mouse or rat or there is a smell attracting them...at least that's what they showed on tv...
I'm a country boy; these big green flys usually come from outside and
they normally last 24 hours before they die inside. Hope you don't have anything that is manifesting them internally.
Anyway, Happy New Year!!
Ah, sir you must be having dogs poo around your house. Get them cleaned.
The green flies in your house is from the outside. They are green because thier body is always in the sun and when their body meets the sun, it turns green like the Statue Of Liberty.