I believe you are correct in that when a certain number is saved the rapture will take place. A lot of events have taken place on feast days. Why couldn’t the rapture happen on a feast day? God would know when the number would come up and make sure it would happen on a feast day.
You’re right. God already knows the date on which the rapture will happen. But the Jewish feast days have both commemorative and prophetic significance to Israel. Since Israel was never promised a rapture, having it occur on a feast day wouldn’t mean anything because it carries no theological importance for them. Their origin, their relationship with God, and their eternal destiny are all different from those of the Church. This is simply an attempt by some Christians to predict the date of the rapture in advance.