불금 is relatively new slang word which is short for 불타는 금요일. In English, people say 'TGIF,' which is an acronym for Thank God It's Friday. People express their joy in knowing that the work week is over. 불금 can be used in various ways. You can express how happy you are because it's Friday by saying "불금이다!" "It's Friday." or "TGIF!" Or you can use the word with the verb '보내다' which means to spend time or money. "불금 보내~" it simply means "Have fun!" And of course, you can only say it on Friday.