Most downpours begin slowly. The wind gushes; the clouds roll in. First it sprinkles. And then, steadily, the rain becomes heavier and heavier. This, however, was not how the water of Noah's flood began.
Noah's flood came upon the world suddenly. And although there were warnings, the pre-flood world trotted on, taking no concern of the coming judgment— they were eating, they were drinking, they were marrying, they were being given in marriage, until the day that Noah entered the ark, and the flood came and destroyed them all. (Luke 17:27)
It came about after the seven days, that the water of the flood came upon the earth. (Genesis 7:10)In Genesis 7:4, God told Noah that in seven more days He would send rain upon the earth for forty days and forty nights. Doubtless, Noah spent those last seven days making final preparations. Once the seventh day arrived, Noah and his household boarded the ark— and on that very same day, the flood began (Genesis 7:13).
In the six hundredth year of Noah's life, in the second month, on the seventeenth day of the month, on the same day... (Genesis 7:11)The calendar in use at that time was obviously not identical to the (Gregorian) calendar in current use. James Ussher, in The Annals of the World, calculates that this date occurred in the year 2349 BC (Sunday, December 7 on the Julian Calendar).
The important thing to notice is that the book of Genesis clearly places the flood in history. The flood is not a mere symbol, nor is it a myth. The flood was an actual historical event that occurred in a certain year, in a certain month, and on a certain day (for more information on the state of the world at this time in history see State of the World (2349 BC): The Flood).
State of the World (2349 BC): The Flood (Part 1) - Genesis 7:11
A Short Account of the Ark's Final Preparation - Genesis 7:6-9
7 More Days - Genesis 7:4-5
The Pre-Flood World: Filled With Violence (Part 1) - Genesis 6:11-13