The phrase and the exact same word that appears in Acts 2:1 is in when the time had come. It shows it as being fulfilled, it shows it as already having happened. So it is the same word. It is the same present tense articular infinitive form as in Acts 2:1. And remember that that indicates contemporaneous action that is not complete. It is happening right now. That is, within the context. It should be translated, And it came to pass when the days were being fulfilled.
Now, I will give you another little proof that this is correct. What chapter in the book of Luke does this appear in? It is chapter 9. How many chapters does Luke have in it? It has 24. How long was it before the days were fulfilled? It was 15 more chapters before the days were fulfilled that He was tried, crucified, buried, and resurrected! See, that He should be received up. It was not fulfilled yet. It was in the process of being fulfilled, that is all. So only when that final process took place where He was tried, crucified, buried, and resurrected would the days be fulfilled.
Those translators should have known better than to translate it as they did. And so Acts 2:1 provides no Bible authority for observing Pentecost on a Monday, the 51st day of the count beginning with the Sunday following the weekly Sabbath between the two holy days of the Days of Unleavened Bread.
