Get the flagpole with the last two digits of your time being the same number.
The number of fireworks that will appear depend on which number was the same.
For example, "11" will result in a single firework, while "22" will show two 
fireworks, etc. Additionally, certain numbers will also give you a bonus item,
as follows.

Fire Flower: 33 or 44 
Ice Flower: 55 
Star: 88 or 99 
Super Acorn: 77 
Super Mushroom: 11 or 22 
Tiny Mushroom: 66