I think what you are saying only makes sense in the context of when to celebrate Easter. There was a dispute for many years about that. In the end, the decision was to focus on Friday as the day on the cross and Sunday as the day of resurrection. The text itself leaves the matter open.
I think you're correct that those are the decisions that were made. But I think the text speaks for itself when put into the context of the early Christian church observing the biblical holy days.