In his work, The King James Only Controversy, James White attempts to show that the translation of 'Easter' is incorrect.
Once again, all White shows is his own poor scholarship.
The day after Passover was always considered to be another feast, not Passover itself.
See Lev.23:6, Nu.28:17, Nu.33:3 and Joshua 5:10-11.
No New Testament verse disproves those verses, which is what White would have you believe.
So, the KJB translators rightly used 'Easter' in Acts 12:4. knowing that 'passover' would be incorrect based on Acts 12:3.
Easter could refer to a pagan holiday or to a Christian one, as Philip Schaff notes in his History of the Christian Church, early Christians did celebrate the Resurrection day,
'The Paschal controversies of the second century related not to the fact but to the time of the Easter festival' (History of the Christian Church, Vol.1, p.480)
Schaff believes the celebration went back to Apostolic age.
So, either reason or even both, may be true regarding the use of 'Easter' for Acts 12:4