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>> “Get more than a 100 miles from Israel without modern communication and nobody would ever know when the feast of trumpets was.” <<
Not exactly true.
They built signal fires in preparation, and lit them when they saw the nearest fire in the distance, and this was done for hundreds of miles after the Maccabee revolt ran the Helenists back to Turkey.
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Well the point is still made. Without modern communication nobody except in the immediate area would know when the feast of trumpets started. That's why God put it in the bible and didn't depend on Jewish tradition.