Two friends responded to my musings about the ides of March. One posted her comment to the blog itself -- one comment is recorded below, with the author's permission. Thanks to both for adding to my store of knowledge. Stay tuned for more about Ides, Kalends, and more than one way to mark the passing of time.
"Hi, Anne --
Well, the Ides are only the 15th of the month in March and three other months. I can't recall the other three right now! The Ides are the 13th on other months -- maybe June, too? I think that's confusing enough for anyone, and people can't remember when the Ides are and decide it's not worth it!
I don't know how the Romans were able to follow the calendar, anyway. You dated things by the Kalends (the first of the month), the Nones, and the Ides (which both varied according to the month -- because one varied, the the other varied). Any given day was marked as three days before the Nones, never the day after the Ides. "After" didn't count. Only "before" counted, or the exact day. Actually, the day before was the "pridie," or "day before" the Nones,Ides, or Kalends. Since the Kalends and the Nones happened before the Ides, which fell either the 13th or the 15th, the bulk of the month was counted as 12 days before the Kalends of the NEXT month. Boy, that's confusing.
Did I tell you about the the time in Latin class when our teacher had us write our birthdays in English and translate them into Latin? My birthday really IS the Kalends of September (Sept. 1), and Kalends is literally the only day that's easy to figure out in Latin. The next day she gave the quizzes back with a stern lecture accusing us of cheating: "I can't believe how many people were born ON THE KALENDS," and she particularly glared at me!
I think the answer to your question is that Shakespeare was very brave to know and use the Ides at all! Everyone else was rightfully too chicken!