No one knows whether Jesus was born on Tabernacles and that does not seem to matter at all. If it did I have no doubt God would have clearly and unequivocally published the exact date in His Word.
And not knowing the feast days will not cause anyone to be "left behind" unless they do not already know the Savoir to whom the feast days simply pointed.
"And beginning at Moses and all the Prophets, He expounded to them in all the Scriptures the things concerning Himself." (Luke 24:27)
I do not have to build a tent in my back yard to know the true tabernacle of God come down from heaven, or blow a ram’s horn once a year to know he will return "at the last trumpet". These are not hidden things.
They were still "virgins", (Christians),
Actually they were not. For the parable concludes, " Afterward the other virgins came also, saying, 'Lord, Lord, open to us!' But he answered and said, 'Assuredly, I say to you, I do not know you.' Watch therefore, for you know neither the day nor the hour in which the Son of Man is coming." Jesus knows all His own by name. The kingdom is full of both wheat and tares, and the foolish virgins were not His own. (We also need to be careful about assuming a word means something without carefully examining the context.)
I was taught this very way in my early walk. I know believe differently. There are many that claim Christ who don't have a clue. They are not going in the Rapture. That doesn't meant they aren't saved. Don't mistake Salvation with Raptured. They are separate. There will be many left behind that will have to choose to refuse the "mark" or not. We are in the age of Grace right now. After the Rapture, we may very well have to chose To give out lives to be saved. Remember, Jesus said "If you deny me before men, I will deny you before the Father." During Tribulation, if you refuse the mark, you will be killed. You will have to declare your allegiance to The Beast to live.
Like you, I was taught the foolish virgins weren't Christians. If they weren't Christians they wouldn't be virgins or in the other parables, servants. What Jesus was telling you was that warming a pew isn't going to cut it. There are many "Christians" that have claimed Christ but have no evidence. I am not the judge to decide salvation for someone, but God is most capable of deciding who is doing His work and who isn't. God doesn't force Himself on people. Just as in this conversation, you are free to believe what you want, and so am I. Who will God chose for "His Bride", the one doing His bidding and longing for His return, or the one that claims Christ, but slumbers and gets drunk thinking they have it made and are going with Him anyway when He comes. I'm not preaching works, just the motivativation of your heart. I used to have a notion that God was a communist and gave equal rewards to everyone. I no longer feel that way. You can be "saved" and still be judged with the smell of smoke on your garments. That doesn't mean you are doomed to hell. You are still saved, but your reward is different that others.
I believe the Rapture could happen tomorrow and many churches wouldn't even realize anything had happened. Even the pastor may be so "luke warm" and "double minded", he may be left behind. Notice I said "he". Many of the lady pastors are the ones preaching God loves sodomites and baby murder. The same people have dedicated their lives to God and believe they are good stead with God. They will even deny the Rapture because they are still here. When the Antichrist comes to place his mark, what will they do? Choose, but choose wisely. You can still get to heaven, but you must literally give your life or freedom to get there.