I'd time it right so that I'd say "knock knock" and right after somebody says "who's there?" the world would explode.
Or I'd just do the shave and a haircut.... world ends.
But for the 2 days in advance I'd blast my memoirs into outer space on a radio, have some sex, get drunk, do some weird drugs, and start a thread on here letting you all know the world's gonna end. But I wouldn't want to waste too much time on my computer, so the thread would say "World ends in 2 days suckers, bye."