Nobility were more likely to keep track of birth dates.
Bernard Plantapilosa was mentioned briefly once, as the brother of William of Septimania; their mother, Dhuoda, wrote a book of advice for her sons, the Liber Manualis. William did not do so well in his life; Bernard, as well, did not have a stellar career.
Even Bernard's nickname refers to appearance rather than actions. We don't know when he first earned the nickname Plantapilosa, which comes from Old Aquitainian and means "Hairy appearance," but it stuck.
While he was Margrave of Septimania, he married and had a son, William. William had a more distinguished career than his father, but that's not why I mention him. He was born on 22 March 875.
Did the Middle Ages think it interesting to have father and son sharing a birthday? Did they take note of coincidences the same way we do? Is there a good reason for both men to be born on the same date? Is there some significance that the birthdays are on the Vernal Equinox? Nine months prior to the vernal equinox is the summer solstice. In 9th century Francia, did christians still see June 24th, Midsummer's Day, as a time for celebrations?