Wi-Fi signals transmit along radio waves at a frequency of around 2.4GHz in most cases, so anything that transmits a radio signal around this frequency can cause signal interference. Many old Christmas lights have a little chip for blink control inside each individual light that emits just this kind of signal — new, more recently manufactured Christmas lights don't have this problem as often.
Wired played around with this thesis and found that the slowdown caused by Christmas lights is barely noticeable, if not totally negligible. Still, if you get paranoid that your snaps aren't downloading fast enough and need a quick fix, you could try moving the lights farther from the Wi-Fi router, switching the router to a 5GHz frequency band, or putting down your phone and enjoying Christmas with loved ones.
Christmas lights aren't the only culprit. There are a host of other devices that could get in the way of your holiday Netflix binge through radio frequency interference.
"These devices include microwave ovens, cordless phones, Bluetooth devices, wireless video cameras, outdoor microwave links, wireless game controllers [...] and so on," according to a Cisco whitepaper. "Even bad electrical connections can cause broad RF spectrum emissions."
Luckily, the Christmas lights and Wi-Fi combination probably won't be too bad — after all, if you didn't notice a difference in a house with a microwave and an Xbox, it's unlikely your temporary lights display will cause you much grief.