The entire action of James Joyce's Ulysses takes place on one day, June 16, 1904. It starts at dawn, with Stephen Dedalus having breakfast with Buck Mulligan and Haines, and finishes up in the early hours of June 17, with Molly Bloom's soliloquy. Joyce picked this particular date because it was the day on which he first walked around Dublin with Nora Barnacle, with whom he would spend the rest of his life.
Joyceans and others celebrate Bloomsday in Dublin on June 16th of each year, re-enacting the events decribed in the book. For most, this involves getting dressed up in period attire, eating and drinking the same meals as Bloom has in the book, and tracing the route he took around Dublin on the day.