what important temperatures does the celsius scale use for a basis?

The important temperatures that the Celsius scale use for a basis are the boiling point of water (i.e. 100°C) and the melting point of ice (or the freezing point of water, i.e. 0°C).

Temperature scales....

Indeed, Celsius used 0C as the freezing point of water and 100 as the boiling point of water. Interestingly enough, the Fahrenheit scale was also set up with temperatures of 0F and 100F, except 0C was the freezing point of a saturated brine solution (NaCl solution) and 100F was the normal human body temperature. This is why the normal temperature today is close to 100 (98.6F). So why is our average body temperature not 100? Either the scale was off a bit, or people may have been running chronic low-grade fevers. Remember, in Fahrenheit's day (1724) health care wasn't what it is today. There is nothing special about the numbers 32 and 212 other than that is where the freezing point and boiling point of water happened to land.

Absolute zero.