The speaker is trying to tell us that he see's a painted window.That the window he see's is painted as a sunset. Than he say's that the painting in the window is just like a girl shining. He see's a woman like a star that set's at rest, that let her self go at night.
He see's a woman clothes glow, just like a sunset below horizon. A woman or a sunset at rest waiting for love. But than realize's that everything he see's is an allusion.
Than he compares the sunset to a woman, saying my best girl/best sunset. A girl/sunset/planet leaving. Than he say's do you leave to your night of rest and from that night may the your windows get darker.
In the first stanza its mentions a "painted oriel" a painted oriel is a painting in a window and he's saying the painting looks like a sunset. In the third stanza it says "like a fair lady at her casements,shines", what that suppose to mean is that theirs a lady/sunset shining in the window.
He compares a lady to a sunset, saying he's in love and that they go but they never stay. He saying that the a lady and or sunset's are beautiful and that they always go down at night to rest so they leave him. To return to him the next day with an all new powerful glow.