When were the First and Second Welfare Theorems proven? What years and by who were the first and second welfare theorems first proven?
Perhaps you have heard the competitive equilibrium referred to as the Arrow-Debreu equilibrium.

The idea of the theorems existed earlier but Arrow introduced the theorems formally in a way that is equivalent to what we today call the first and second welfare theorem in 1951 in his paper

AN EXTENSION OF THE BASIC
THEOREMS OF CLASSICAL
WELFARE ECONOMICS.

Debreu is more associated with proving that the equilibrium exists under certain conditions. If you think about it this makes the first welfare theorem much stronger. His seminal paper on the subject is

The Coefficient of Resource Utilization.