Bloomsday (Irish: Lá Bloom) is a commemoration and celebration of the life of Irish writer James Joyce, observed annually in Dublin and elsewhere on 16 June. The day is named after Leopold Bloom, the protagonist of Joyce's 1922 novel Ulysses, the events of which take place on Thursday, 16 June 1904. Joyce chose to set his novel on this date as it was the date of his first sexual encounter with his wife-to-be, Nora Barnacle.

The English compound word Bloomsday is usually used in Irish as well, though some publications call it Lá Bloom (Bloom's Day, in Irish).

The first mention of such a celebration is to be found in a letter by Joyce to Miss Weaver of 27 June 1924, which refers to "a group of people who observe what they call Bloom's day – 16 June".

On the 50th anniversary of the events in the novel, a Wednesday in 1954, John Ryan (artist, critic, publican and founder of Envoy magazine) and the novelist Brian O'Nolan organised what was to be a daylong pilgrimage along the Ulysses route. They were joined by Patrick Kavanagh, Anthony Cronin, Tom Joyce (a dentist who, as Joyce's cousin, represented the family interest) and A. J. Leventhal (a lecturer in French at Trinity College Dublin). Ryan had engaged two horse-drawn cabs, of the old-fashioned kind, in which in Ulysses Bloom and his friends drive to Paddy Dignam's funeral. The party were assigned roles from the novel. Cronin stood in for Stephen Dedalus, O'Nolan for his father Simon Dedalus, Ryan for the journalist Martin Cunningham, and Leventhal, being Jewish, was recruited to fill (unknown to him, according to Ryan) the role of Leopold Bloom. They planned to travel round the city through the day, starting at the Martello tower at Sandycove (where the novel begins), visiting in turn the scenes of the novel, ending at night in what had once been the brothel quarter of the city, the area which Joyce had called Nighttown. The pilgrimage was abandoned halfway through, when the weary pilgrims succumbed to inebriation and rancour at the Bailey pub in the city centre, which Ryan then owned, and at which in 1967 he installed the door to 7 Eccles Street (Leopold Bloom's front door), having rescued it from demolition. A Bloomsday record of 1954, informally filmed by Ryan, follows this pilgrimage.

Since 1994, the Bloomsday Festival has been celebrated in Dublin. The Bloomsday Festival is one-week long and is scheduled on the week of June 16th. The festival involves a range of cultural activities, including Ulysses readings and dramatisations, pub crawls and other events. Enthusiasts often dress in Edwardian costume to celebrate Bloomsday, and retrace Bloom's route around Dublin via landmarks such as Davy Byrne's pub. Hard-core devotees have even been known to hold marathon readings of the entire novel, some lasting up to 36 hours. The Bloomsday Festival is organised by the James Joyce Centre on behalf of the city of Dublin.

The James Joyce Tower and Museum at Sandycove, site of the opening chapter of Ulysses, hosts many free activities around Bloomsday including theatrical performances, musical events, tours of the iconic tower and readings from Joyce's masterpiece.

"Every year hundreds of Dubliners dress as characters from the book ... as if to assert their willingness to become one with the text. It is quite impossible to imagine any other masterpiece of modernism having quite such an effect on the life of a city."

On Bloomsday 1982, the centenary year of Joyce's birth, Irish state broadcaster RTÉ transmitted a continuous 30-hour dramatic performance of the entire text of Ulysses on radio.