Today, February 24th, is the feast day, in Orthodox and other Eastern-rite churches, of the First and Second Finding of the Head of St. John the Forerunner i.e. the Baptist. Roman-rite martyrologies from at least the ninth century through to the modern Roman Martyrology prior to its revision of 2001 entered under today a commemoration of the Finding (later, the First Finding) of the Head of John the Baptist. Until 1970 today was the feast day of St Matthias the Apostle, as it still is in the Usus antiquior calendar.
In Greek tradition the First Finding took place in the time of the Emperor Constantine the Great (306-337). In the Latin tradition represented by the later ninth-century martyrology of Usuard of Saint-Germain the Finding took place in the time of the Emperor Marcian (450-457); this corresponds with the customary dates for the Second Finding (either 452 or 453).
As Lady Bracknell might well have observed, to lose it once might be considered unfortunate, but to lose it twice betokens carelessness.
My posts from last August for the feast of the Decollation of St John the Baptist can be seen at The Decollation of St John the Baptist and Relics of St John the Baptist. The head itself has become such a prized item that there are have been several claimants over the centuries to be the actual relic.
With acknowledgements to John Dillon's post on the Medieval Religion discussion group for today.