"Bloody Sunday" occured on Sunday, February 23rd 2014, the eleventh day of the Gen 1 TPP playthrough. Day 11-12 saw many major events in the playthrough occur, most notably the capture of Battery Jesus in the Power Plant and the accidental release of 12 Pokémon, for which the day got it's notorious nickname. This day of the playthrough was spent mostly in the Power Plant.
The Capturing of Zapdos
There had been many posts in the subreddit and discussions in major chat channels in the days preceding entry to the power plant indicating that a strong strategy that would benefit gameplay in the future would be to capture a Zapdos. The entire side-quest to the power plant, of course, had the purpose of at least engaging the most useful of the legendary birds.
There was much division over whether this could even be accomplished in the chaotic stream of commands but the strong pro-anarchy sentiment insisted it was possible and probable with the aid of the Master Ball. Most agreed that Zapdos was a worthy use of this device. With the Master Ball being one of the few usable items towards the top of the inventory, the opportunity was deemed low risk.
The EncounterSome time around the 10d 07h 44m mark, RED did battle with Battery Jesus. Bird Jesus was sent out - this proved worrisome, as an Electric attack from Zapdos could easily spell the end of the Pidgeot should the strategy fail.
The stream made no attempts to attack Zapdos. Instead of attacking, the players made their way into the item menu in a hopeful sign.
After several attempts, players were finally able to select the Master Ball around five minutes into the battle. Zapdos was caught and nicknamed "AA-j". With a forboding whisper, Zapdos was immediately sent to the PC due to the party being full.
The full extent of what this meant was overshadowed by the celebration that followed the battle. Many saw this as a realization of what was slowly solidifying into the Helixian psuedo-religion that touted anarchy as the pure version of the stream. Supporters of democracy quietly looked on in surprise.
Attempt to retrieve AA-j from the PC
Approximately one hour after his capture, players accessed the Power Plant PC to attempt to replace a Pokémon in the party with him. The stream was in Anarchy mode at this time. Players first deposited Air Jordan, followed by Bird Jesus approximately a minute later. At the 08h 22m 05s mark, Zapdos was withdrawn from the PC. The goal of the players was now to fill the last spot in the party, however, due to the mechanics of Anarchy mode, the following Pokémon were accidently released: A (Venonat), B-!) (Venonat), AIA (Exeggcute), AAA (Paras), Venonat, Nidoran, X(araggbaj (Cabbage), DUX, Geodude, H (Nidoran), AEWWW (Nidoran), and AAJST(???? (Dig Rat). This is still the largest single day release of Pokémon in TPP history. The final line-up of the party after using the PC was Bird Jesus, Dash Bat, Air Jordan, The Keeper, Rick Gastly, and Burger King.
The Loss of DUX
Due to the difficulty in teaching a Pokémon an HM in Anarchy mode, DUX was the playthroughs only Pokémon with the Cut ability. Because of his release, players had to re-enter the Rock Tunnel, which they already had travelled through before in complete darkness.
Origin of the name "Bloody Sunday"
The origin of the name "Bloody Sunday" can be easily attributed to the great loss of Pokémon occurring on a Sunday, but in reality it likely goes deeper than that. Bloody Sunday is a name for several historical events, notably a demonstration in Northern Ireland in 1972 where the Brittish army ended up killing 14 civilians, that bear that name for a reason; the first known use of 'Bloody Sunday' to refer to the PC Massacre was by Twitch user Gordzing
The phrase "Bloody Sunday" also has assonance in many English dialects.