The how is not so important, just the bare fact that these lights are threaded into the peace process that followed a long battle. For a while they were on every night—glowing, festive, perfect. And after a while they were not turned on every night, because things like that do not always become habit.
What has become habit: if someone is not home yet, if a visitor is expected, if someone in the house feels festive (or wants to feel festive) these lights are on. It has never been discussed, never been planned. Just happened.