Acts 20:7 KJV
(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.
This text should read 'one of the sabbaths' not first day of the week.
G3391
μία
mia
Thayer Definition:
1) only one, someone
Part of Speech: adjective
A Related Word by Thayer’s/Strong’s Number: irregular feminine of G1520
This is the meaning of the the word first it means one not first.
Matthew 5:24 KJV
(24) Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift.
G4412
πρῶτον
prōton
Thayer Definition:
1) first in time or place
1a) in any succession of things or persons
2) first in rank
2a) influence, honour
2b) chief
2c) principal
3) first, at the first
Part of Speech: adverb
A Related Word by Thayer’s/Strong’s Number: neuter of G4413 as adverb (with or without G3588)
Citing in TDNT: 6:868, 965
Notice the different word this word means first not the other which means one.
G4521
σάββατον
sabbaton
Thayer Definition:
1) the seventh day of each week which was a sacred festival on which the Israelites were required to abstain from all work
1a) the institution of the sabbath, the law for keeping holy every seventh day of the week
1b) a single sabbath, sabbath day
2) seven days, a week
Part of Speech: noun neuter
A Related Word by Thayer’s/Strong’s Number: of Hebrew origin H7676
Notice the word for week.
So the translation should read 'one of the sabbaths' not first day of the week.