If I had to pick a day & time to have a church service, Sunday AM would not be my first choice; I'd prolly go with Thursday night. I mean, sleeping in on a Sunday morning is amazing! [there's no mystery to why so many people do it: its pretty sweet!] Of all the times and all the day, why would Christians have picked Sunday morning as a time/day to gather together? Didn't they know sleeping in on a Sunday morning is as close to heaven as some people will ever get? perhaps you know what I'm talking about?!? [when we lived in Kentucky, we attended a church that had a Saturday night service, it was awesome!]
So why Sunday? Christians talk about the Sabbath, but let's just clear something up: the Sabbath is Friday at sun-down to Saturday at sun-down. Christians gathering on Sunday has very little if nothing to do with the Jewish Sabbath. Its a completely different day. Sorry, but we don't meet on Sunday because of Sabbath.
If you read the book of John, you'll notice that John makes a small note about the specific day that Jesus is raised: he calls it, "the first day of the week." Its a very powerful metaphor [think the first week in Genesis 1; John's book is LOADED with imagery. Its so brilliant]. You see in the 1st century Jewish mind, Sunday was the first day of the week; its much like Monday is for us today. There was no concept of a weekend, Sunday was the beginning of a work week. When John says that Jesus rose on the first day of the week, he's making a statement about a new start. This event has begun a new reality, a new world, a new order is among us. He was the first to emobdy CHANGE you can believe in.
This is one of the earliest understandings of salvation, a whole new order to creation. In this reality, Christ is the center of the world. This new week is no longer ordered by the old way, but by the reality of Jesus' death and resurrection. Life not death has won. No longer must we live stuck in our evil ways. No longer is death in the lead, no! life has won. It is this reality that Christians would gather to celebrate [this is why the would begin their gatherings with the greeting, "He is Risen!"]
Now the earliest Christians would have had to work on Sunday - there was not a nice 2day weekend. If you were a Jew you took 1 day [the Friday night to Saturday night Sabbath] then you went back to work, and the Romans didn't have such a day. They still had to work on Sundays. So they would get up early, before they went to work, to worship the new reality: DEATH is dead, and Jesus Christ is alive! They met, as we do, to celebrate the beginning of a new week.
Why do we meet on Sunday mornings? Because its the first of the new week. Its a tangible [touchable/observable] image of who we are as Christians. Because of this, we wake up and celebrate, because a new week is upon us and death is dead.
HE IS RISEN!