The first section is called Silly Sisters, a reference to June Tabor and Maddy Prior's folk music.
"There was a green bough hanging on the door." A fast search to see whether this is a custom didn't turn up anything, but I found this: "If I keep a green bough in my heart, the singing bird will come." (Chinese proverb) I have no idea whether it's an actual Chinese proverb, but it's a good thought.
"The year was old, and turning lightward, into winter." I don't know whether the actual date will turn up in the text, but this matches the Delaware/Philadelphia climate I'm used to. The hottest part of the summer and the coldest part of the winter come after the solstices.
"Cold and waning....Ariane looked back the way she'd come". One of the big themes I want to look at is resemblances between the two women and the two goddesses. I'm not going into more detail because I'm trying to avoid spoilers.
"hill beyond blue glaze of hill"-- this is more a thing I see with mountains, but they do look like layers of blue glaze.
"While we poor wassail boys do trudge through the mire...the wind. Cold by the door, it sang."
"Should I turn my coat? Or break a twig of holly? Or snare a wren?" Folklore: Turning your coat can be a protection against pixies. The Wren and Its Folklore.
Speaking of the generally tricky and unreliable nature of things, these links are the result of fast google searches. I don't guarantee the reliability of any of it, and welcome corrections and further discussion.
"Her air of gravity and desolation was, she knew, rather spoiled by her wraggle-taggle trail of clutter." Sylvie keeps an eye on herself in self-obstructive way. If she's neurotic, can her parallel goddess also be neurotic?
Folksong: the Raggle Taggle Gypsy. I'm reasonably sure this is just picking up the cool-sounding adjective rather than a deeper reference to the text of the song.
King Herla's rade-- King Herla was one of the leaders of the Wild Hunt. The link about him looks as though there's some fine eerie legendry, but I'm going to try and get this entry finished. If you know of any fiction about being led on the Wild Hunt through references rather than woods, please led me know.
"distraitly wandering through time" This definitely happens to one of the other characters.
"An owlish, cat-stumbling sort, but her absurdity did not console her." I don't know whether it's reasonable to expect anyone to be consoled by their absurdity, but it's a good phrase. It might also be worth keeping an eye out to see whether there's any consolation by absurdity later in the book.
"the kettle withering on the fire" another good phrase
"backed into the drying rack, which collapsed into the ashes like a fainting stork, all legs, and plumed with tatty underwear" another good phrase, and a reason for doing this sort of close reading-- until I transcribed it, I only noticed the fainting stork, but missed the way the metaphor was continued.
"She began to feel quite happy, a cup unspilled, a new moon turned." Ariane gets a lot of lunar metaphors.
"--yet they were drifted over, rooted in the sweet nourishing decay of loved ephemera" This is the only book I've seen make bad housekeeping look numinous. It also seems to me that the house would not survive so much neglect, but there's more to fiction than meticulous world-building.
I'm not transcribing the whole passage, but there's a nice parallel between the house and a forest, with the tall furniture standing unchanging while the soil is building up around it. It's probably relevant that the owner of the house is named Sylvie.
Ariane remembers the rooms, but they're a little different: ..."only subtly aslant: a table moved, a book left open. The dissonances made an eerie harmony of parallels, fifths, and seconds." Probably a reference to rules for musical harmony. It's probably relevant that parallel fifths are discouraged in some kinds of classical music, but normal in some kinds of folk and medieval music.
Incidental search result: There's an Etsy shop called Moonwise which has good-looking jewelry.