While Christmas lights do contribute to the electromagnetic noisiness that can interfere with your home Wi-Fi, they're not the worst offender.
That would be your neighbors, whose routers run on the same wireless channels as yours.
Microwaves put out the same 2.4 GHz radiation as many older routers, too.
Don't forget the very walls of your home: The thicker they are, the harder it is for Wi-Fi to get through.
And even your own body, 60 percent of which is water, can absorb Wi-Fi.