Because the Jews say that’s when it is, and I follow that Jewish custom.
I do occasionally find myself conflicted with some elements of Torah, such as Urim and Thummim, which I view as soothsaying. Or certain elements of animal sacrifice during the Old Temple period, and its possible restoration in The Temple to come. I try to keep kosher, though limiting it to poultry and fish for similar reasons.
In other words, I sometimes argue with God, which is the original meaning of Israel...as opposed to islam, which is to submit.
It took a long time in many Christian settings to realize that I was much happier with an Old Testament outlook.
Though I realize too that the United States could never have been founded without Christianity, so I recognize that for a great deal of people there is happiness therein too.
What I do lament however, is how many Christians seems to give The Law short shrift believing that the New Testament outlook somehow abrogates The Law. I obviously do not adhere to such a view.
“Because the Jews say that’s when it is, and I follow that Jewish custom.”
Modern day Jews. But you have no idea if it was changed since Moses brought down the tablets. The belief isn’t based on the Bible but on modern-day Jewish custom.