The day of the week observed for rest and worship. Most Christian traditions observe the Sabbath on Sunday. Judaism — along with some Christian traditions such as Seventh-day Adventists — observes the Sabbath on Saturday. (Jews’ observance of the Sabbath begins at sundown Friday [and is often referred to by the Hebrew word Shabbat].) Capitalize in religious references but lowercase when talking about periods of rest.
REFERENCE: Religion Stylebook