They didn't. Read carefully, and understand Hebrew culture:
The Havdala service is an Hebrew custom to extend the joy of the Sabbath, and to usher in the new week. It happens at sundown after the Sabbath ends and is done often in home settings - It is the origin of 'pot luck', with everyone bringing a little something, gathering together to eat supper. Bread and wine feature prominently in the blessing before the meal, and speakers stand to preach during the dinner...
Re-read the NT with that understanding, and the whole thing changes.
What we find in the Bible are the practices and commands given to the churches by the apostles whom Jesus chose and equipped to teach on His behalf. One of these practices was assembling on the first day of the week to partake in the memorial feast, etc.
Now we can call it “Firstday” if the more common “Sunday” offends us. But whatever we call it, it’s the day after the Sabbath. The Sabbath is the seventh day, and there are seven days in the week. So the early disciples, under the direction of Jesus’ own apostles, assembled on the first day of the week, just as we read about in Acts 20.