It depends on what kind of a disaster it is.
On one level, if one takes the example of Bhopal Gas leak? The obvious answer is the gas leak is the disaster itself, but the beginning of the disaster can perhaps be attributed to a lot of factors that came together and enabled that moment of disaster. The government policies that enabled the establishment of the factory knowing the kind of substances that are to be produced, the complete faith in the techo-scientific aspect of the establishment of the factory as a step towards development, the capitalist mechanisms that failed to ensure safety regulations by cutting corners, the proximity of the factory location to the residential areas, etc These in some way anticipate the disaster and therefore the beginning of the disaster could be traced before the actual event. The disaster and often times the way it is managed bring upon a continuing series of disasters, as the systems get overstretched and the susceptibilities get overexposed. Often time the lack of will to build inclusive infrastructures that ascertain and guarantee basics to everyone are writ off as being too expensive and have to be traded in for options and solutions that favour a few.
Therefore, does the disaster end is an interesting question. More than 30 years after the tragedy the victims of Bhopal Gas leak continue to feel betrayed by the series of events that transpired, decisions that were taken on their behalf without their consent. For some, invariably, the disaster is ongoing in terms of its impacts on their everyday lives.
The commencement of any disaster is a matter of debate since there are disasters which are measured on the visible destruction, for example, the Haiti earthquake, then there are some that become a matter of conscious, famines when we talk about that, then there are some which will have no underlying symptoms but will create havoc, the 2008 financial crisis. Also, it becomes a matter of privilege to when any disaster in a real sense begins, the plague wasn't really a concern till it reached Europe. And like any receding tide, it ceases to exist when the privileged are no longer affected.
At a conceptual level, every disaster has a making. Chernobyl wasn't one day in the making, it was a result of scientific negligence on part of the state. It became a disaster the day ideology trumped science, the blowout was the result of the making.
At the same time, any disaster continues to exist in forms and ways which are mostly not visible to the naked eye. Bhopal Gas Tragedy leaves every new generation with lifetime health issues, therefore we cannot claim that the tragedy ceases to exist. The trauma and scars that are left are often the markers of the perennial reminder of the disaster.