That's a good question. The answer is that the time between the two events will vary for different observers, however the property that light can't travel fast enough between the events is invariant and can be agreed upon by all observers.
It's somewhat important that this property remains invariant, simply because all observers should be able to agree whether a flash produced at one point in spacetime is visible at another point in spacetime.
How does this work, if you can spare a hand-wavy explanation? If the perceived time between events varies by observer, and if for every observer there is a precise threshold at which events become casually disjoint, naively I would imagine there to always be disagreement.
Just as there is a difference in perceived time there's also a difference in perceived distance. As it turns out these cancel out in a way that ensures the speed of light is the same for all observers. Together with the axiom that everyone agrees whether something travels in a straight line you can figure out most of special relativity.
It's somewhat hard to explain why this works without handwaving or just pointing to the maths, but the youtube channel minute physics at least has some good visualizations: https://www.youtube.com/watch?v=Rh0pYtQG5wI
It's somewhat important that this property remains invariant, simply because all observers should be able to agree whether a flash produced at one point in spacetime is visible at another point in spacetime.