It's from the Anglo-Saxon Chronicle:
A.D. 793. This year came dreadful fore-warnings over the land of the
Northumbrians, terrifying the people most woefully: these were immense sheets
of light rushing through the air, and whirlwinds, and fiery, dragons flying
across the firmament. These tremendous tokens were soon followed by a great
famine: and not long after, on the sixth day before the ides of January in
the same year, the harrowing inroads of heathen men made lamentable havoc in
the church of God in Holy-island, by rapine and slaughter. Siga died on the
eighth day before the calends of March.
"sixth day before the ides of January" is assumed to be a scribal error because of other sources and because Vikings wouldn't raid in January. So, it's assumed to be June, "the ides" is the 13th, and the 6th day before it, presumably, would be the 8th.
https://www.gutenberg.org/cache/epub/657/pg657.html