"Mary, Peter and Josh went to the store to buy some bread."This is how I would write that sentence.
"Mary, Peter, and Josh went to the store to buy some bread."Basically, a serial comma is a comma that immediately follows the penultimate item in a series. According to my English teacher, it is not good grammar to use them. This prompted me to do a little Wikipedia reading and credible source follow-up. It is conventional to use serial commas in everyday writing (which explains why I use them impulsively), but they are less common in British English and in journalism. We just had a writing test at school with a possibility of an editorial or article format, so that is probably why my teacher brought it up; nevertheless, I like serial commas. I remember eating out at a restaurant once and looking at a menu that read, under the specialty sodas heading, "grape, cherry, orange and cream". Since menus sometimes leave out conjunctions altogether, I didn't know if it meant there were orange sodas and cream sodas, or if there was a particular soda that was both creamy and orange-y. In a final testament to the ambiguity that abounds when serial commas go missing, there's somewhat of an urban legend of a writer who dedicated her book "to my parents, Ayn Rand and God."
In other news, apparently my school library has to throw out a ton of books that don't have recent enough copyright dates, yet some of them are classic fiction books that have been out of print for ages! This is absurd! I rescued a box full of them today and will probably go back for more, but too many books will still tragically go to waste. At least I got my own copy of Heart of Darkness out of it.