“The End Of The World” is a vague concept. It can cover anything from society slipping into a dark age to global thermonuclear war to the sun exploding and a sudden universal vacuum collapse. Consequently your (“you” can be either a random individual or a major national government) response to a foreseeable End will vary considerably. When it comes to Hollywood, most Ends are preventable (send a team of badasses to fight the aliens or blow up the comet) or avoidable or just survivable. But for this thought exercise, assume a End is on the horizon that can NOT be avoided or survived. Options include:
1) The Sun’s gonna splode. Nowhere in the solar system to escape to.
2) Incoming neutron star or multi-solar-mass black hole will pass inside of one AU from the sun, tossing every planet, moon and asteroid out into the dark, while passing close enough to Earth to tidally disrupt it
3) Message in the cosmic background radiation is decoded giving a hard cutoff date for this particular simulation.
4) Whatever. In five years, humanity is extinct and there’s no avoiding it. No heroics or glorious programs will help.
So, let’s say The End has been detected, checked and confirmed. The experts have run the numbers every which way and there’s no getting around the fact that we’re boned. In fiction, the default position of a government that is aware of this sort of thing is that they will do what they can to keep the facts as quiet as possible as long as possible, to keep people from panicking and to keep the economy humming along. But in the latter case, the necessity of that is to allow The Messiah Project, whatever it is, to be funded and built. If society collapses, then the government can’t build the Mars colony ship or the underground bunkers or the planetary deflector shield or whatever. But in this case, everyone with sufficient clearance to know about The End knows that no Messiah Project will amount to a hill of beans. So in this case, what is your advice to the President? Keep the secret so that the public can go on for a little while longer in ignorance and relative peace, or spill the beans and let people know the truth? Does it matter if society goes down tomorrow if it’s going to be wiped out in five years anyway?
From the other end: you’re a regular schmoe, and the government announces that in five years, The End. The Russians concur, the Chinese, the Brits, the Canucks… every government has their experts check the data and they all conclude that five years and then that’s it. What do you do? Go to work tomorrow or decide to retire (knowing that if too many people choose Option B, then the economy collapses overnight)? Go Purge and Festival? Or eat a bullet?