I agree with Trevor, that the sun and the moon represent the good and evil. I think he sun is good because in pg. 144 I think it's Koroviev that mentions that the sun is setting so it's time to go. I think that now that the "good" is tucking away, they say it's time to go because the moon will be out representing evil where Koroviev and Behemoth will be out performing their evilness.