A lot of this fits the battle of the Pelennor fields: "At last the sun is shining, The clouds of blue roll by" - as Sauron's army and influence advanced the sky darkened and when he lost this battle it became light again. But a lot doesn't fit to that particular battle/book, including the part about the angels of Avalon, as Avalon was not from Tolkien's world but the legends of Merlin and King Arthur. The song is not completely about that battle but there are references to Lord Of The Rings
things like Ringwraiths and most of the song can be interpreted to be about it if you choose.
Caleb - Christchurch, New Zealand