Interesting - as soon as I read the thread title, that’s the seven days I thought of.
Christianity - particularly Pauline Christianity - has some positives over other contenders: no ethnic distinctions, low barriers of entry (don’t have to get circumcised), and ample promised rewards (in the afterlife). Not sure which other religions might apply - Buddhism, perhaps? Of course, it would have to be something that Constantine knew about…
I do kinda like this one - especially if you move it to the entire week of 28 June - 4 July, when Kaiser Bill decided to give the Dual Monarchy his blank check. War wasn’t inevitable before then; it pretty much was thereafter (although the Czar had a chance to keep it localized as well).
If we go into prehistory, I think the first seven days after the Chicxulub asteroid hit probably qualifies.
But Jesus’ impact was spread over his entire career, which lasted a lot longer than Holy Week. Heck, I don’t think that the Bible even says that the events commemorated on Palm Sunday were a single week before the events commemorated on Easter, so even the portion of his career in Jerusalem could have been longer than a week (and really, he spent his entire life not far from Jerusalem, which was the biggest city in the region, so he’d probably been there before then anyway).
More generally, there are doubtless many major moments in history which have never been marked. Just for a currently-relevant example, we have no idea who Patient Zero was for covid, and we can only make imprecise estimates as to where and when they were. But despite our lack of knowledge, there still was a Patient Zero, and that’s had some very large effects.
I’m not much for the Great Man theory of history, but it’s certainly true that some assassinations led to huge changes.
In the U.S. alone, I’d argue that Lincoln’s assassination outdoes Kennedy’s in terms of after effects, and I might make a case for McKinley’s as well. But none of those really changed the world in major ways.
I still believe the 1918 Pandemic is the prime contender. It was unusual in that it killed people in the prime of their lives. The flu typically kills people under 5 and over 75, in contrast to the 1918 Spanish flu which killed mainly people 20-40.
How many of the ~50-million who died from the Spanish flu would have changed the world in significant ways? How many of the >500-million infected had their lives and work forever incapacitated?
As for defining the momentous watershed week, perhaps the week the H1N1 virus mutated and became virulent to humans (it began as an avian gene). Or the week patient zero went rogue.