I'll borrow from www.wikipedia.com to give the brief explanation of Bloomsday:
"Bloomsday is a holiday observed annually on June 16 to celebrate the life of Irish writer James Joyce and commemorate the events in his novel Ulysses, all of which took place on the same day in Dublin in 1904. The day is also a secular holiday in Ireland. The name derives from Leopold Bloom, the protagonist in Ulysses, and June 16 was the date of Joyce's first outing with his wife-to-be, Nora Barnacle, when they walked to the Dublin village of Ringsend.
The event is commemorated with a range of cultural activities including academic conferences, Ulysses readings and dramatizations, pub crawls and general merriment. Enthusiasts may often dress in Edwardian costume to celebrate Bloomsday. The first celebration took place in 1954 and a major five-month-long festival (ReJoyce Dublin 2004) took place in Dublin between April 1 and August 31, 2004. On the Sunday prior to the 100th anniversary of Bloomsday in 2004, 10,000 people in Dublin were treated to a free, open air, full Irish breakfast on O'Connell Street consisting of sausages, rashers, toast, beans, and black and white puddings, and a pint of Guinness."