Q. Why did Christianity abandon Shabbat and focus on Sunday?
A. The short answer is that the early Christians maintained Jewish practices for a lengthy period including the 7th-day Sabbath. In time they marked their separateness from Judaism by requiring their adherents to celebrate the 1st day of the week as the Lord’s Day, i.e. the day when Jesus was said to have risen from the dead. Sunday thus became the touchstone of Christian faith and identity.
Various Church edicts defined the way in which Sunday was to be observed. Paradoxically, Judaism was criticised for Sabbath strictness while Christianity introduced its own strictures and produced a day that was often heavy and humourless, lacking the Jewish concept of Oneg Shabbat.
It is interesting that Jewish communities in many Diaspora countries, careful not to offend their Christian neighbours, avoided weddings and other celebrations on Sundays.