There are some pretty adventurous claims out there, about the Roman holiday of the “dies sanguinis” or “day of blood”. This article from About.com is rather better than most, in that it is referenced, but it includes one of the odder claims I have seen:
In ancient Roman history, the 24th of March (VIII Kal Apriles) was the Dies sanguinis ‘day of blood,’ possibly a precursor of Good Friday.
Today I have been attempting to find out what, if anything, the ancient sources actually tell us. I even looked in the RealEncyclopadie in vain.
In the Chronography of 354, part 6 (the Philocalian calendar), I recall an entry for the 24th March, IX kal. April. — sanguem.
22 H D A XI ARBOR·INTRAT 23 E B X TVBILVSTRIVM 24 F C IX SANGVEM DIES·AEGYPTIACVS 25 I G D VIII HILARIA
Web searches suggest a festival of Bellona. Others suggest that this is the day on which the priests of Cybele castrated themselves. So … what are the facts?
Looking at Duncan Fishwick, “The Cannophori and the March Festival of Magna Mater”, Transactions and Proceedings of the American Philological Association, Vol. 97 (1966), pp. 193-202, we get something on p.201:
The earliest direct allusion to the dies sanguinisis in connection with the death of Marcus Aurelius in A.D. 18o (Tertullian, Apolog. 25), but a passage in Valerius Flaccus (ob. A.D. 92 or 93) seems to make clear reference to the sanguinary rites of the day as early as the Flavian period (Argonautica. 239-42):
sic ubi Mygdonios planctus sacer abluit Almo
laetaque iam Cybele festaeque per oppida taedae
quis modo tam saevos adytis fluxisse cruores
cogitet aut ipsi qui iam meminere ministri?
With this may be compared a text of Martial (ca. A.D. 40-104) suggesting that the lavatio served also to purge the instruments used on the dies sanguinis (3.47.1-2):
Capena grandi porta qua pluit gutta
Phrygiumque Matris Almo qua lavat ferrum.
OK. Let’s turn those quotes into English. Tertullian, Apologeticum 25:5 is online here.
 Why, too, even in these days the Mater Magnahas given a notable proof of her greatness which she has conferred as a boon upon the city; when, after the loss to the State of Marcus Aurelius at Sirmium, on the sixteenth before the Kalends of April, that most sacred high priest of hers was offering, a week after, impure libations of blood drawn from his own arms, and issuing his commands that the ordinary prayers should be made for the safety of the emperor already dead.
Valerius Flaccus, Argonautica, book 7 contains this:
And just as the anger of the mournful Mother29rends every year the frenzied Phrygians, or as Bellona lacerates the long-haired eunuchs,…
29. Cybele mourning for Attis; Bellona, goddess of war, whose priestesses and votaries, eunuchs called Bellonarii, cut themselves with knives at her festival (Juvenal, 4. 123; Lucan, 1. 565).
But book 8 is our reference:
So when the holy Almo washes away Mygdonian sorrows,10 and Cybele now is glad and festal torches gleam in the city streets, who would think that cruel wounds have lately gushed in the temples? or when of the votaries themselves remember them?
10. The festival of Cybele, the Great Mother, on March 27th (Ovid, Fasti4. 337); the image of the goddess was washed in the Almo, a tributary of the Tiber.
Martial, book 3, epigram 47:
Yonder, Faustinus, where the Capene Gate drips with large drops, and where the Almo cleanses the Phrygian sacrificial knives of the Mother of the Gods, …
Michelle Salzman’s On Roman Time is accessible to me and page 167 says:
The mourning became more violent on the following day, 24 March, Sanguem, when the devotees flagellated themselves until they bled, sprinkling the altars and effigy with their blood. This was also the day when certain devotees of the goddess, carried away by their emotion, would perform self-castration. During the “sacred night” of the twenty-fourth, Attis was ritually laid to rest in his grave and the new galli were inducted into the priesthood(presumably symbolizing the god’s rebirth); at dawn, then, a day of rejoicing Hilaria could begin.
Note the lack of footnotes, tho.
And so it goes on. How do we know that this day is associated with these events? Which source says so?
I suspect that we are looking at the backwash of some early 20th century textbook, in which the statement was made as a theory to explain these references, and has thereafter been taken as fact. Perhaps it is sound. Perhaps not. It would be interesting to know its origins.