I don´t know if there is a exactly meaning in this song, but in this song Anastasia sings about her first walz and that she will see her grandma in December again.
This is what I think, so it may not be true, but anyway, I think that it's about her life a long time ago in a party in December. It's basically about her life when she was young.
To me, this song is overflowing with nostalgia. Anastasia remembers something, she just doesn't know exactly what it is. It's a beautiful song about a memory she doesn't quite understand.