The term ‘day of the Lord’ is associated with a period of time when God intervenes directly with mankind. There are over 80 references to this time throughout the Bible. On Patmos, God sent an angel to John revealing what must happen during the time leading up to, including, and after Jesus returns to the earth. This is the time the Bible refers to as the day of the Lord. The term ‘Lord’s Day’ appears only once in the Bible. It comes after John explained he was receiving a message from an angel, John tells us he was having a vision on the Lord’s Day. It describes the time the vision depicted, not the day of the week he was having it. The phrase ‘Lord’s day’ didn’t become synonymous with a day of the week for another century or two.

Revelation 1:1 New Testament for Everyone
1 Revelation of Jesus the Messiah! God gave it to him to show his servants what must soon take place. He signified it by sending a message through his angel to his servant John,

Revelation 1:10 New King James Version
10 I was in the Spirit on the Lord’s Day, and I heard behind me a loud voice, as of a trumpet,

Except for those in Alexandria and Rome, most of the first century church observed a Saturday Sabbath, according to the Roman historian Salminius Hermias Sozomenus and 5th-century church historian Socrates Scholasticus. By the fourth century though, those who kept the Sabbath were branded as Judaizing heretics. Sunday worship was embraced when Constantine made Sunday a day of rest from labor on March 7. 321 AD. Church leaders saw this as a positive step as it enabled more people to easily attend services. 

Believing Jesus rose before sunrise on the first day of the week, the majority of Christians came to know Sunday as the ‘Lord’s Day’. Wanting to avoid following the Jewish determination of Passover that used the lunar calendrical system of the Bible, the timing for Easter was arbitrarily prescribed in 325 AD at the Council of Nicaea and was based on mathematical principles instead. Probably wanting to avoid the Jewish persecution of the past, early Christian leaders, with the backing of Rome, fashioned their own days of worship. Most Christians today don’t question why, or when those holidays came into effect. The Bereans of Paul’s day did not follow easily. They searched the scriptures daily to prove what was right. Do you have the same desire?

2 Timothy 2:15 King James Version
15 Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.

More biblical information links.