Sunday, just as it is now. See the related question below.
First day of the week.
John 20:19 mentions that the disciples were assembled together on the first day of the week with the door locked for fear of the Jews when Jesus suddenly stood among them. Acts 20:7 mentions that Paul preached to the disciples when they came together on the first day of the week to break bread. Those are the only mentions of gatherings on the first day of the week. Most other references to the first day of the week were regarding Mary Magdalene and the other Mary finding Jesus' tomb empty very early on the first day of the week.
In the King James translation, the word pair "first day" appears 48 times: 36 times in the Old Testament (where it most often has to do with something God commands the Israelites to do on the "first day" of a given month), and twelve times in the New Testament, where 6 of the 12 refer to the "first day" of the week, on which Jesus was resurrected.
He rested on the 7th day, which was the Jewish Sabbath. It was then and still is Saturday. On the morning of the first day of the week we are told the women came to the tomb and found that Jesus had risen. That would have been Sunday. See the related question below.
In this question, never by name.
The first day of the week is Sunday.
It is called Palm Sunday.
Holy Week is the seven day period before Jesus was resurrected. It begins on Palm Sunday - the day which Jesus rode into Jerusalem.
The previous day was the Sabbath, so they couldn't do any work that day.
The New Testament gospels record that Jesus ministered every day of the week.
It is often said that Monday is the first day of the week, but it is only the first day of the working week, not the actual week. Sunday is the first day of the week. See the related question below.
Sunday is the first day of the week but Monday is the first day of the work and school week.