coven

Reader's Digest 10月号の記事 "The Wiches of Sunset Valley" から引用します。
It was Halloween, my favorite time of the year, and I was on my way to visit my mom, who had suffered a debilitating stroke. I arrived dressed as a witch, not quite sure if it was to lift her spirits or my own. Mom laughed, and I felt every second of it. I didn't want to detach from that positive note, so I remained in costume even when it was time to leave. I noticed some buses passing on their way to the nearby elementary school. I waved, they waved back, and so it began. Over the next 12 years, a coven grew. Now it includes mothers, grandmathers, and great-grandmothers. We gather our broomsticks, cauldron, and various other decorations to set the scene in Mom's front yard.

"coven" の意味を調べます。
・Oxford English Dictionary: A group or gathering of witches who meet regularly.: ‘On the first day of spring in 1996, our local newspaper ran an article about a local coven of witches.’

・Vocabulary.com: A group of witches is called a coven. In books, a coven of witches often gather at night to make potions and cast spells.
The mythology of witches has them meeting under cover of night, often in mysterious groups of 13. Although there are only three of them, the "weird sisters" in Shakespeare's play "Macbeth" could be considered a coven. To people who follow the contemporary religion called Wicca, a coven is a gathering, no different from a congregation in Christianity. The word coven arose in the mid-1600s, ultimately from the Latin root conventus, "assembly."
ハロウィーンの衣装をまとって集まるので "coven" が使われたのですね。