At what temperature are Celsius and Fahrenheit equal?

At −40°. That's the only point where °C and °F have the same numerical value: −40°C = −40°F.

What is absolute zero?

Absolute zero (0 Kelvin, −273.15°C, −459.67°F) is the theoretical minimum temperature, at which particles have minimal thermal motion. It has never been fully achieved experimentally.

Why does the US still use Fahrenheit?

The US never transitioned to the metric system during the international standardization efforts of the 1970s. Fahrenheit was proposed by Daniel Fahrenheit in 1724 and became entrenched before metrication.