There are reasons why biblical events happened on certain dates.
It was not a coincidence that Jesus died on Passover day (Nisan 14). He was the sacrificial lamb of the New Covenant, as we read in Matthew 26:27–28:
And he took the cup, and gave thanks, and gave it to them, saying, Drink ye all of it; For this is my blood of the new testament, which is shed for many for the remission of sins.
It was not a coincidence that Jesus rose from the dead before sunrise on Nisan 17. Jesus was fulfilling His prophesy, as we read in Matthew 12:40:
For as Jonas was three days and three nights in the whale’s belly; so shall the Son of man be three days and three nights in the heart of the earth.
It was not a coincidence that the Holy Spirit descended on the Apostles on the day of Pentecost. Many people were in Jerusalem on that day to witness God’s gift to the Apostles, as we read in Acts 2:1–5:
And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance. And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven. Now when this was noised abroad, the multitude came together, and were confounded, because that every man heard them speak in his own language. And they were all amazed and marvelled, saying one to another, Behold, are not all these which speak Galilaeans? And how hear we every man in our own tongue, wherein we were born?
In my opinion, it was and still is a bad idea to observe Good Friday as the day Jesus died and Easter Sunday as the day Jesus was resurrected. Good Friday and Easter Sunday have no Scriptural support. As for Pentecost—the fiftieth day after Passover—it was the day that God gave Moses the law on Mount Sinai. How can Pentecost be seven weeks from Easter Sunday?
Other than for people’s convenience, I see no reason why we should observe Good Friday, Easter Sunday, and Pentecost Sunday.