I see here that you brought two different unrelated points

Your question is: What is the Christian’s Day

The answer is: Sunday

Why

The answer is: Because Jesus rose from the dead on Sunday as the bible says: “When the Sabbath (Saturday) was over, Mary Magdalene, Mary the mother of James, and Salome bought spices so that they might go to anoint Jesus body. Very early on the first day of the week (on Sunday), just after sunrise, they were on their way to the tomb and they asked each other, “Who will roll the stone away from the entrance of the tomb?”

But when they looked up, they saw that the stone, which was very large, had been rolled away. As they entered the tomb, they saw a young man dressed in a white robe sitting on the right side, and they were alarmed “Don’t be alarmed,” he said. “You are looking for Jesus the Nazarene, who was crucified. He has risen! He is not here. See the place where they laid him. (Mark 16: 1-6

So you related the day to a wrong reason, but there is no problem as I want to thank you for this chance to let me tell you about the Holy Light getting out of Christ’s tomb in the Church of the Resurrection in Jerusalem every year on Saturday of the holy week before Sunday, the day of Christ’s Resurrection. I don’t know if it’s allowed to put the links here, but anyone who wants to see the miracle can search for “The Holy Light” or “The Holy Fire”. And not just see but also read what history tells about it

The miraculous event has not only Christian testimonies, but non-Christian testimonies as well. Before entering the Tomb of Christ, the patriarch is examined by Jewish Israeli authorities to prove that he does not carry technical means to light the fire. This investigation used to be carried out by Muslim Turkish Ottoman soldiers

God bless you. I’ll pray for you