This is the martyrdom date in 831 for the iconodule saint Euthymius of Sardis.
Euthymius was just a child when Byzantium’s century-long internal conflict over the image-veneration wrote St. Stephen the Younger into the pages of this here blog way back in 764.
By the time Euthymius attained the bishopric of Sardis in the 780s, the Empress Irene was putting an end to her predecessors’ anti-icon campaigns, and Euthymius took part in the Second Council of Nicaea that made the new policy official.
Posterity has a difficulty measuring by way of scanty and partisan sources the true state of sentiments surrounding icons during this period but it’s a sure thing that for an empire besieged both west and east, religious questions connected inextricably to geopolitical ones. Irene’s shift towards embracing what iconoclasts saw as graven images spanned about a quarter-century which also coincided with humiliating reverses for Constantinople. Irene’s son was thrashed by the Bulgars to whom her treasury was then obliged to submit tribute; then Irene had that very son deposed and blinded. Irene was toppled in her turn by her finance minister but Emperor Nikephoros too was trounced in battle and his skull wound up as the Bulgar Khan’s ceremonial goblet.
Small wonder that when Leo the Armenian took power in 814 he reflected that
all the emperors, who took up images and venerated them, met their death either in revolt or in war; but those who did not venerate images all died a natural death, remained in power until they died, and were then laid to rest with all honors in the imperial mausoleum in the Church of the Holy Apostles.
For a prelate like Euthymius, this meant a return to the opposition benches. He’s reported to have been arrested and exiled twice in the ensuing years before finally being scourged to death in 824 at the behest of Leo’s successor; however, scholarship has better associated this event with the more vigorous anti-icon persecutions of Theophilus after 829. In 831, Arab forces devastated Cappadocia and also captured Panormos in Byzantine Sicily. In light of these reverses Theophilos discovered that an anti-iconoclast manifesto predicting the emperor’s imminent death had been circulated — so again the link between prestige abroad, sedition within, and those damned icons. Theophilus attributed the pamphlet to a pro-icon bishop named Methodius, who was a friend of Euthymius, and had both men arrested.
Imprisoned on the island of St. Andrew, near Constantinople in the Sea of Marmara, the two men were questioned about their associates by the postal logothete — probably Arsaber, the brother of [anti-icon future patriarch] John the Grammarian — who was accompanied by the chartulary of the inkpot Theoctistus. Euthymius seems to have mocked Theoctistus and would name only one of his visitors: Theoctista, the mother-in-law of both the logothete and the emperor!* Theophilus had both Euthymius and Methodius beaten soundly. While Methodius, who was just over 40, could endure it, the 77-year-old Euthymius died from his injuries on December 26 and became an iconophile martyr. The empress Theodora was reportedly so upset at Euthymius’s death that she told Theophilus that God would desert him for what he had done. (Source)
The History of Byzantium podcast covers this period in episode 103.
* Theoctista was an actual iconophile. Her house in Constantinople later became the Monastery of Gastria — and post-1453, a mosque.