Here's how to convert Fahrenheit to Celsius:

Formula:

Celsius = (Fahrenheit - 32) * 5/9

Calculation:

Celsius = (66 - 32) * 5/9

Celsius = 34 * 5/9

Celsius = 18.89 (rounded to two decimal places)

Therefore, 66 degrees Fahrenheit is approximately 18.89 degrees Celsius.