Sabbath, New Moon, & Feast Days

A Biblical “day” begins and ends at sunset, rather than at midnight.
In Genesis 1:5 [KJV], the story of creation, it says,
“And the evening and the morning were the first day.”
From this, we conclude that a day begins with evening, that is, sunset.
Sabbath, New Moon, & Feast Days
Begin the evening before the date specified and
end at nightfall of the date specified,
that is, at the time when it becomes dark outside, about an hour after sunset.
Leviticus 23:32b [KJV]
“…from even unto even, shall ye celebrate your sabbath.”