When did Y!A change "Best Answer" to "Favorite Answer"?

Back in Oct when the new format was rolled out. It makes more sense, since often the answer selected by the OP isn't the "best" (or even relevant). By calling it the "Favorite" it doesn't confuse people who come here looking for actual information, and finding the question they had has already been asked/answered (but the answer chosen by the OP may not be the most correct/best answer provided).

About 3 months ago, i hadn't noticed it for ages so it could have been 4 months ago. 

It happened a few months back during the turn over of the new site.  Verizon Media does not inform us of their changes so it hard to say but it was sometimes at the beginning of Oct or Nov.

Ohhhh, at least 2 months ago......I noticed it sometime before  Christmas. 

When they made a New Format they changed it to Favorite Answer

Because they finally decided to change the name to reflect the reality.

Many people select AN answer to meet their bias, these answers are rarely the "best", so the socially conscious Yahoo! staff have changed it to "Favorite".

I never noticed the change, it does make sense.

About 2 weeks ago, or beginning of the year.

Sorry, 2 months ago, not 2 weeks.

It was probably changed because people will often pick their favorite answer even though it may not be the best answer.