If a person were to change religions, say from Islam to Judaism it makes sense that one would willingly change from worshipping on Friday to Saturday. However, one would have to have an incredible powerful revelation (knowledge) to remain a Muslim yet change to a different day of worship. For a religious person to make such a change would make an interesting yet isolated story. But now consider hundreds of people simultaneously changing, not their religion mind you, but the day of worship. This would be a bit more than interesting.
And this is exactly what the early Jewish followers of Jesus did, practically overnight. Shortly after the death of Jesus, they instantaneously began to gather to worship the same God (still very much Jewish) on a different day. From Saturday to the first day of the week (Sunday).
To make matters more interesting, it was not as if those Jewish Jesus followers had joined a group that was already worshipping God on Sunday. No such group existed. No, they continued believing in the same God, but the Empty Tomb had proven to them (given them knowledge) something so powerful about their God that they instantly began worshipping Him on a different day (They would come to call it the Lord's Day).
Isn't it reasonable to believe that something incredibly powerful must have occurred after the death of Jesus to cause this monumental change? There is only one logical conclusion to explain how hundreds and then 1000s of devoted Jewish believers could have changed a sacredly held, 4,000 year old worship practice overnight. They witnessed God do something so astonishing on the first day of the week, shortly after the death of Jesus, that it warranted a new day of worship. Only seeing first hand the Empty Tomb and the Resurrected Jesus would have had that kind of power. From Saturday to Sunday, this is the Big Switch.
He is alive.