Scripture, at least twice, tells us our Lord was crucified on a Friday.
"Then he took [the body of Jesus] down and wrapped it in a linen shroud and laid Him in a tomb cut in stone, where no one had ever yet been laid. It was the day of Preparation, and the Sabbath was beginning." - Luke 23:53-54
That really ends any possible debate there could even be. Jesus was crucified on the day before the Sabbath, on the day when Jews were preparing for the Sabbath to begin, that's Friday, specifically Friday before sunset. It was nearing sundown on Friday, because it says "the Sabbath was beginning"--the Sabbath begins sundown Friday and ends sundown Saturday.
Our second text is also from Luke's Gospel,
"And He said to them, 'What things?' and they said to Him, 'Concerning Jesus of Nazareth, a man who was a prophet mighty in deed and word before God and all the people, and how our chief priests and rulers delivered Him up to be condemned to death, and crucified Him. But we had hoped that He was the one to redeem Israel. Yes, and besides all this, it is now the third day since these things happened." - Luke 23:19-21
"The third day since these things happened"
Sunday = Third day
Saturday = Second day
Friday = First day
Jesus died and was buried before sunset on Friday. Jesus spent the entirety of the Sabbath in the grave. And then early Sunday morning He rose from the dead.
Christians celebrate the Lord's resurrection as the Christian Passover, called Pascha in Greek. Early Christians were divided in their customs on whether to celebrate this Feast at the same time as the Jewish Passover (Nisan 14th), a position known as Quartodecimanism (literally Fourteenism); or whether the Feast should always be celebrated on the Lord's Day (aka Sunday). Over time the Quartodeciman position became less popular, and appears to have largely only been retained by a handful of schismatic and heretical groups when at the Council of Nicea, as a periphery issue, resolved to standardize the timing of the Feast.
The gathered bishops came to agree that it would be a good thing if Christians were celebrating the Paschal Feast together, at the same time, and so a method of calculation, which was already in practice in the Church of Alexandria and the other Egyptian churches. This method of computation has continued to be how we do things.
In English-speaking places we usually call the Paschal Feast by the name "Easter", which goes back to when the Anglo-Saxons who invaded Britain were converted to Christianity, and they began to refer to the Paschal Feast by the name of the month it occurred in: Eostermonath; thus referring to the Feast as "Eostre". The only similar thing we see is that in standard German the Paschal Feast is called by the cognate, Ostern. However in some parts of Germany and in regional German dialects the word Paasken (Pascha) is preferred.
The Paschal Feast is the day on which Christians have celebrated the resurrection of our Lord Jesus Christ. It is the oldest day on the Christian calendar, and probably emerged among the first Christian communities even during the lifetime of the Apostles--the use of the word Pascha (Passover) clues us in on this. And common sense would tell us that the most central and important event in the Christian religion would be the thing to be given its own celebration.
Overtime the entire week leading up to the Paschal Feast became important, Holy Week or Passion Week, beginning on the day we call Palm Sunday, and concluding on Holy Saturday, also called the Great Sabbath (for Christ our Lord was at rest in His tomb). Between Palm Sunday and Holy Saturday we also remember the Institution of the Holy Eucharist (Maundy Thursday) and the Friday of His death, Good Friday.
Historically and biblically we know the Lord was crucified on a Friday, we remember this and observe this every year as Good Friday. And the Feast of the Lord's Resurrection, the great and holy Feast of Pascha, the Day of Days, is the day we celebrate and declare with our loudest song, praise, and words: Christos anesti! Alithos anesti! Christ is risen! Amen He is risen!
-CryptoLutheran