Allah (SWT) is saying that when the humans feel that they own the Earth and can do whatever they wish or come to their mind, that's when the Hour or Judgment Day takes place. Since Allah (SWT) knows the future and the exact minute of the Hour, why did He say "There reaches it Our command by night or by day". Why didn't He say" There reaches it Our command by Night ", or why didn't He say "There reaches it Our command by Day ". Why did He say " by night OR day ".
The reason the Almighty Allah (SWT) said " by night OR day " because if the Hour would to take place during the day time in America, then it will be night time in Australia, and vice versa. Judgment day happens to all earth regardless if it is a night or a day time. That's why Allah (SWT) said "night OR day". This is a scientific fact that Earth does rotate around its self for us to have the day and night.