I have done my shared of studying this subject as well.
I first found that "easter" in Acts 12:4 is the Greek word πάσχα, Latin Vulgate used pascha, which is translated as Passover everywhere but in this one KJV verse. How odd for them to do that. If there was a new celebration, with a different word, would not the Greek Bible have a new word for it?
Some one was defending the word "Easter" being put here instead of Passover. As a defense for believing that Easter was the correct word, and not Passover, was that verse 3 in Acts says "then were the days of unleavened bread". And because of that it could not be referring to Passover, (because according to the OT, Passover would have done passed) because no where in the Bible does it refer to Passover as the days of unleavened bread.
BUT...
This is where you have to be careful and not believe what others say ( you should ALWAYS do your own research). In fact the New Testament DOES refer to the Passover as the "days of unleavened bread"
Mat 26:17 Now the first day of the feast of unleavened bread the disciples came to Jesus, saying unto him, Where wilt thou that we prepare for thee to eat the passover?
Here we have on the 1st day of unleavened bread, the disciples asking Jesus where does He want them to prepare passover.
Mar 14:1 After two days was the feast of the passover, and of unleavened bread:
Mar 14:12 And the first day of unleavened bread, when they killed the passover,...
Luk 22:1 Now the feast of unleavened bread drew nigh, which is called the Passover.
I can't remember it all, but I did research and the Jews did at some point started referring to one as the other. (I think is was during or after the Babylon capture, this is when they create a new set of rules base on Gods commandment, and put more burden on the people. I suggest studying this, some things that seem confusing, because the OT says this, and the NT seems to contradict are cleared up because the Jews changes.) Which made it a bit confusing at first. This is why I believe that knowing Jewish history (not just from the OT but from Jewish sources) helps a lot in understanding the Bible.
So, why would someone translate passover as easter? So I tried to see when perhaps it was first translated that way, and I went all the way back to Luther's translation of his German Bible (1522). He translated passover as Ostern. But, he translated ostern in other places where the word passover is found as well, like in Mat 26:2
William Tyndale's 1534 Bible used "ester" in place of passover. (If memory serves me right, Tyndale used Luther's writing and manuscripts to translate his English Bible)
But, John Wycliffe Bible 1382 (the first ever English translation, hand written, the printing press had not been invented yet), translates the pascha in Act 12:4 as pask.
So atm it looks like it is something that Tyndale may have started. It just needs further research.
Jesus told His disciple exactly what to do in His remembrance: And it wasn't celebrating His resurrection.
Luk 22:19 And he took bread, and gave thanks, and brake it, and gave unto them, saying, This is my body which is given for you: this do in remembrance of me.
1Co 11:24 And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me.
1Co 11:25 After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me.
Hope this helps. If you do research and find any other info, please share.