On June 24, or St. John’s Day, early Britons lit chains of huge fires to support the diminishing sun.
These fires were fed with the clean bones of dead farm animals and were called “bone fires,” which evolved into the word “bonfires”.
There were bone fires, wood fires, and a mixture of both wood and bones was called a “St. John’s fire”.
The name was given, naturally, to the fires that burned heretics at the stake.
A large, controlled fire is commonly called a “bonfire”.