There is no direct statement in Scripture that says Jeroboam, king of the northern ten tribes of Israel, changed worship from the seventh day to the first day of the week. However, there is plenty of biblical proof that he did so.
When the twelve tribes of Israel split into two kingdoms, Jeroboam became fearful that his people would eventually reunite with the tribes of Judah, the southern kingdom. He said to himself, "Now the kingdom may return to the house of David: If these people go up to offer sacrifice in the house of the LORD at Jerusalem, then the people will turn back to their lord, Rehoboam king of Judah, and they will kill me" (I Kings 12:26-27).
Jeroboam knew that if his people considered Jerusalem to be their religious center, they would soon return their allegiance to the house of David. To retain control over the people, Jeroboam devised his own religious system. He made two idols—setting them up in the towns of Dan and Bethel, which he made the centers of worship—hired the lowest of the people to be his priests, and created a counterfeit Feast of Tabernacles for the people to observe in the eighth month, not the seventh, as God had ordained in Leviticus 23:34 (I Kings 12:28-33).
The people followed him and served the pagan god Baal (II Kings 17:16). The word Baal means "lord," and he was worshipped as the lord of the sun. It should be no surprise that the sun-god was worshipped on the day we now call Sunday. This can be verified in any reliable reference work on the subject, such as Webster's Rest Days. Sunday, of course, is the first day of the week.
The fact that Israel turned to Baal worship is proof in itself that the nation began to observe the first day of the week. Did they also forget
Some 200 years later, they were still observing the first day of the week. God warn in Hosea 2:11, "I will also cause all her mirth to cease, her feast days, her New Moons, and her Sabbaths—all her appointed feasts." He condemns Israel for keeping their own, not His, days of worship. There is no doubt as to the real source of the days Israel kept: "I will visit punish her for [observing] the days of the Baals to which she burned incense" (verse 13).
Because of Jeroboam's actions, Israel abandoned God's Sabbath for the pagan "Lord's Day," the day of the sun-god Baal, and never returned to keeping the true Sabbath. The evidence is that Jeroboam made the day of Baal the national day of worship. Notice this brief summary of his actions: "Then Jeroboam drove Israel from following the LORD, and made them commit a great sin" (II Kings 17:21). Jeroboam is the only king the Bible records to have "made Israel sin" (same verse, and 20 additional passages). He deliberately and actively turned Israel away from the true worship of the true God, doing away with the seventh-day Sabbath by instituting Baal worship. Ezekiel says this sin, idolatry combined with Sabbath-breaking, was the primary cause of their defeat and captivity (Ezekiel 20:1-24; see Nehemiah 13:15-22; II Kings 17:5-18).