Why is it entitled the day after tomorrow?

Answer 1

The day after tomorrow= 2 days in the future.

If in the movie, the world were going to fail in one day, then it might simply be called "tomorrow", or possibly something with more creativity.

However, it failed within two days. Thus the title.

Answer 2

The title "The Day After Tomorrow" refers to the timeframe in which the film's events take place - a climate change disaster that unfolds rapidly, leading to extreme weather conditions. The phrase implies immediate consequences and urgency, highlighting the immediacy of the threat posed by global warming.