Christians attend church on Sundays, they sing, listen to a pastor speak, etc. When did such a concept first appeared? Did the early Christians practice this? or is this 'attending church' a medieval tradition from the Roman Catholic church?
In the earliest days of the church Christians met in the Temple in Jerusalem and in each others homes. (Acts 2:26)
The Apostles would preach in the Jewish synagogues, often evangelistically, but sometimes even at the invitation of the other Jews. (See Acts 13 for example)
They would meet in lecture halls, such as the Hall of Tyrannus in Ephesus. (Acts 19:9)
The New Testament doesn't mention purpose built church buildings, and in the New Testament era the most common meeting place for Christians was in each others' homes.
The earliest known church building is the Dura-Europos church, which was converted from a regular house in around 233-256AD.
I presume you mean something separate from preaching in Jewish synagogues (which Acts describes) or gatherings in house-churches (which Acts also describes).
You can see from the list of oldest known churches that dedicated buildings for Christian gathering were quite old, preceding the Middle Ages. You will see that there are few churches (that we know of) that were built before the mid fourth century - this is largely because it would have been illegal before the Edict of Toleration (and unlikely before the Edict of Milan). The list includes one church from 52 AD, founded by St. Thomas (the Apostle) in India.
This was a Comment but seems relevant to the question.
Saturday is actually the Sabbath day, the day in which God Rested.
Sunday is the first day of the week.
Sunday was the traditional day for Studying God's word.
They didn't attend church on Saturdays because they rested on that day and attended church on Sunday for study and fellowship (mostly fellowship) this is why they met in other people's homes.
Believers came together in homes and temples and fields on the day they would have normally been in Synagogues and other places of learning (places teaching about Jewish religion). They did this to learn about the things that Jesus was all about, that is the reason for "Church". this is one of the stepping stones into why we have "Church" and "Church buildings", and why we go to "Church"
the thing of it is that they really had this ritual of Studying on the first day of the week all along, the Christians just decided to do it where the teacher was, when Jesus went to prepare a place for us, the believers of that time decided they needed a more permanent place to fellowship. so really what happened was a change of Religion, not a change in the meeting habits.
even today when new church is formed it usually starts as a weekly meeting of a group of people that meet in each other's homes, and when there are too many people to meet inside of one person's home then they start thinking about buying a building and making a "Church Home" for their group to meet in.
The Meeting has always been the same, the Place is what changes between groups.