I don't know for sure. But just to get the ball rolling, here's a possible theory, given a 30 day month. Early = 1st to 10th. Mid = 11th to 20th. Late = 21st to 30th. Beginning of (included in early) = First 5 days = 1st to 5th. End of (included in late) = Last 5 days = 26th to 30th. More @Wikipedia
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