I used particle core http://particle.io and a relay module http://allegro.pl/show_item.php?item=6573707747 to turn the lights on/off. Relay is needed because you cannot get more than 3,3V from particle core pin and you shouldn't puch much current through it anyway.
The module has a bit surprising API, but thanks to https://community.particle.io/t/switch-relay-by-connecting-pin-to-ground/9077/27 I could use it succesfully.
Christmas lights (in my case 4,5V) are wired to the relay, relay IN is connected to pin D6 on particle and that's it.