Easter is the most important point of the Calendar for Christians.
It is so because if it never occurred then Christianity is a complete fraud.
Jesus could rise from the dead because he was without sin. To do this he had to be God ( without sin) and be man ( represent mankind).
Thus his death on the cross was the punishment for all sinners whom believe in him.
We know this occurred because he walked around for 40 days so we have heaps of eyewitnesses. We also knew this because the Jewish elite who hated Jesus would have been shown him to be dead and thus killed off ( pun intended) this subversive sect as they saw it.
Because of the many eyewitnesses they could not do this.
We also have the radical change in behaviour of his apostles. When he dies on the cross they were scared, hid from the Authorities etc. Once they confirmed the resurrection they completely changed. They were willing to die for him!
One last thing. He dies on Friday the first day and rose from the dead on the third day, The first day of the week ( not the Sabbath) Sunday.
I see some people like to the first day is not the first but day 0.
Sunday is not day 0 of the week but day 1.