Surprisingly, it appears that it is marked by some Christians as well, although in an unrecognizable way.
The 19th century book The Rest of the Words of Baruch: A Christian Apocalypse of the Year 136 A.D states:
Nor were they without some' encouragement to this belief from the actual event. One of the things written across Jewish history was the fatality connected with the 10th of Ab. [sic] We may get some idea of the import of this day by recalling the language of Josephus concerning it: "the fated round of times was come, the tenth day of tho month Lous, on which aforetime' the city had been burnt by the Babylonians" (Bell. Jud. VI. 4. 5). He does not hesitate to say that the time had been calculated by God: "one might rightly marvel at the accuracy of the cycle; for it was the very same month and day on which the city was formerly burnt by the Babylonians" (Bell. Jud. VI. 4. 8).Sure enough, if one looks at the Greek Orthodox Book of Saints (in Greek, naturally), one sees that among the list of saints commemorated on November 4th comes "Narration in the Lamentations of Prophet Jeremiah on Jerusalem."
So deeply was this day marked with black in their calendar that there is reason to suspect that from that day to this it has been kept as a day of mourning both by Jews and Christians. With the Jews, of course, this is obvious: but tho following considerations suggest that the Christian Church also shared this mourning with them. The Greek Church keeps a special memorial of the fall of the city on the 4th of November, and reads on that day, as we shall see, a portion of the very Apocalypse which we are engaged upon. But the question naturally arises as to was how a memorial designed for the Fall of the City came to be read on this date. The answer is that Ab, which is the eleventh month of the Hebrew Calendar, has been replaced by November, the eleventh in the Julian year, while some reason not known to us has displaced the day from the tenth to the fourth. We may, therefore, suspect that Christians as well as Jews concerned themselves to note the fatal day.
(The Rest of the Words of Baruch is also related to Tisha B'Av, as it includes the text of Lamentations but also some text called 4 Baruch that supposedly includes parts of Jeremiah that were not included in the canon. It also includes an early Rip Van Winkle story.)
I will not be posting anything until Sunday afternoon. (But there is no reason for Ian not to post the normal linkdumps in the meanwhile if he wants to.)
Previous Tisha B'Av posts:
A sad anniversary
A reason to keep mourning on Tisha B'Av
Tisha B'Av, 1948
Weeping over the ruins of Jerusalem
The Kotel, 1912
A reason to cry
Judaism's holiest site is being desecrated today
Documentary on Israel's disengagement of Gaza
2013: The Churban underneath the Mount
2014: 2000 years of mourning for the Temple
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