Sunday worship was from the Mithras religion of the Roman army. Jesus was a Jew and worshiped on Saturday.
Any part of a day is counted as a day - not 24 hours
Days not hours
Jesus was raised on Sunday. It became the Lord’s day and the early church worshipped on the Lord’s day.
The idea that the church adopted pagan religious practices is not supported by any historical evidence. “Someone did something on the same day before so you are copying them” is not a valid logical principle. There are only 7 days of the week to begin with. They were led by the Holy Spirit to choose a different day in order to separate themselves from the old system (sacrificial/ Law of Moses) which was passing away.
Sunday was the first day of the week so also fitting for the most important event in history.