Try to put yourself for a moment in the mind of one of the original disciples of Jesus. How would you have been different from what you are today?
Well, in the first place, you would have been a Jew. Like Jesus himself, you would have been a regular in synagogue attendance, and you would have been a Sabbath keeper. The Sabbath, you must understand, was more than just another commandment, more than a mere doctrine. It was a matter of religious identity.
Now how and when did all that get changed? Why would you, at some later time, abandon the observance of the Sabbath in favor of Sunday? And what would have been the consequences of that change?