Act_20:7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.
In our western mindset and understanding, we read the above and immediately think Sunday. But this is not talking about the first day of the week, and taken alone is out of context.
The word "day" is in italics and means the translators added the word "day".
First is "one of" and Week is "sabbaths"
So reading entire context of this passage:
Act 20:6 And we sailed away from Philippi after the days of unleavened bread, and came unto them to Troas in five days; where we abode seven days.
Act 20:7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.
"After the days of unleavened bread" is the counting of weeks each Sabbath till you reach Shavuot/Pentecost.
So Acts 20:7 actually reads "first = G3391"
μία
mia
mee'-ah
Irregular feminine of G1520; one or first: - a (certain), + agree, first, one, X other.
And "week = G4521"
σάββατον
sabbaton
sab'-bat-on
Of Hebrew origin [H7676]; the Sabbath (that is, Shabbath), or day of weekly repose from secular avocations (also the observance or institution itself); by extension a se'nnight, that is, the interval between two Sabbaths; likewise the plural in all the above applications: - sabbath (day), week.
LXX related word(s)
H7676 shabat
H7677 shabbaton
If you do not understand the Feast of Unleavened Bread and how to count to Pentecost, this KJV verse will be read misinpreted and presented falsely as a Sunday. The translators of the KJV wanted to be misleading, but the original language continually gives them grief in their misconception.