There is no one event. There are multiple events bc it is not linear but a cycle that starts the previous year.
Key events:
Egg maturation -Having the raw materials to get the job done happens the previous year or, in some individuals or even populations during tough conditions, the final "deposits" are made during early spring. This is why the larger fish are more apt to spawn earliest -they can afford it. And small fish are most apt spawn latest. I've seen this in my own (albeit amateur) pond studies.
Photoperiod -Photoperiod changes have direct endocrine effects that goad the process and GENERALLY synchonize the event. Lots of literature out there on this and it's powerful enough to experimentally force spawning events in fish and other animals -with other keys in place.
Temperature -Temperature is critical to cold-blooded creatures, and with fish it is the final push. It allows for body weight gain efficiency, activity required for spawning behaviors, and protects temperature sensitive eggs.
Moon Phase -This appears to be a real effect -the full and possibly the new. It seems to be a cue that allows for a mass synchonization, provided the stage is set (the previous three keys): I've recorded years when spawning occurred on temperature swings, falling in between the moons.
The first two of these keys we can't do much about really (unless you are Flukemaster and can fatten your fish on his private lake). But as far as fishing is concerned, temperature and secondarily moon phase are the things to track. And it has worked, like clockwork, for me.
Catt mentions powerplant lakes, and there are also bass planted far south of their natural range. There are lakes in Central America that never cool down much and have flat photoperiods. Those bass can spawn anytime and have adapted their cues to water level changes. They may spawn over a long seasonal window but what stops them is the need for tissue recovery and growth. Supposedly bass do poorly down there and burn out young. Nature finds a way, at least to a certain degree in that living things didn't come from nowhere, they have a history, and that dictates what they are and what they can do.