We'll Meet Again is a love song made famous by British singer Vera Lynn with music and lyrics by Ross Parker and Hughie Charles. It is one of the most famous songs during the World War II era, and it resonated with the soldiers of the Second World War to fight for their families and sweethearts.
In addition to the song's long history of covers and performances on films and TV, Bill Cipher performed a segment of the song in Weirdmageddon 3: Take Back The Falls after he brought Ford out of his golden statue form, only to be chained into a mysterious room, where Bill rose from the ground, playing a piano and singing the song.
The song might also be a reference to 1962 movie Dr. Strangelove, in which the song plays at the endscene, when the world ends on several atomic blasts; Bill Cipher playing it on the piano might symbolize the weirdness Armageddon he wants to create itself.
We'll meet again,
Don't know where, don't know when,
But I know we'll meet again, some sunny day.