When does BC ends and AD begins?

BC ends when AD begins: 1 BC comes before 1 AD. There is no year "0".

"BC" stands for "Before Christ", and "AD" stands for "Anno Domini", which in Latin means "in the year of the Lord".

Scholars prefer to use "BCE" and "CE", which stand for "Before the Common Era" and "Common Era", respectively, to mean the same thing as "BC" and "AD", but do not imply Christian connotation.