For a story that’s about sex six days a week, there’s something prayer-like, even Biblical, about Emma Copley Eisenberg’s “Sundays.” Jeffrey, a scientist, is Mondays and Thursdays, Lamya, a Muslim marine biologist, is Tuesdays and Wednesdays, and polyamorous Beth, who lives in North Carolina with her primary partner, is Fridays and Saturdays. Six days, three different people to love or desire. “Sundays” is a story for which it feels imperative to read every word, and that rewards second and third readings. Eisenberg writes like Grace Paley for 2017, with sentences that are at once compact and airy, assertions that are also questions, and politics that are radical yet tenderly cultivated.