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.
No, the Sabbath is not the only day of the week named in the Bible. Other days of the week are mentioned in the Bible, such as the first day of the week (Sunday) which is referenced in relation to Jesus' resurrection.
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.
Jesus was crucified on a Friday.
Jesus was resurrected on a Sunday.
It is called Palm Sunday.
In this question, never by name.
The first day of the week is Sunday.
Jesus rose from the dead on a Sunday.
Holy Week is the seven day period before Jesus was resurrected. It begins on Palm Sunday - the day which Jesus rode into Jerusalem.