Bloom thinks in Calypso that the "August bank holiday" might be a good time to pay a visit to Milly in Mullingar, because he would have a three-day weekend. Bank holidays are national public holidays in Britain and Ireland. The name is mysterious. Banks do close on these days, but so do most other businesses.
In 1871 an M.P. named John Lubbock pushed the Bank Holidays
Act through Parliament. It established Easter Monday, Whit
Monday (just after Pentecost, which is the seventh Sunday
after Easter), St. Stephen's Day (or Boxing Day, just after
Christmas), and the first Monday in August as bank holidays.
Banks were already closed on other important holidays like
Good Friday and Christmas, and the Bank Holiday Act of 1903,
introduced by an Irish MP, added St. Patrick's Day (March 17).
So by 1904 there were seven bank holidays in Ireland. Today
there are eleven.
In his book At Home (2011), Bill Bryson discusses Lubbock's "breathtakingly radical idea of a paid secular holiday for workers. It is almost impossible now to imagine what excitement this caused. Before Lubbock's new law, most employees were excused from work on Good Friday, Christmas Day or Boxing Day (but not generally both), and Sundays, and that was it. The idea of having a bonus day off—and in summer at that—was almost too thrilling to bear. Lubbock was widely agreed to be the most popular man in England, and bank holidays for a long time were affectionately known as 'St. Lubbock's days.' No one in his age would ever have supposed that his name would one day be forgotten" (519-20). In a footnote to this passage, Bryson observes that "The name 'bank holiday' was an odd one, and Lubbock never really explained why he elected to call it that instead of 'national holiday' or 'workers' holiday' or something similarly descriptive. It is sometimes suggested that he meant the holiday only for bank workers, but that is not so. It was always intended for all." It is a fact, though, that Lubbock himself was a banker.
Gifford notes that the August bank holiday is a "long holiday weekend not unlike Labor Day weekend in the United States." It also is an oasis in a desert, with no other holidays for a couple of months before and after. So it readily suggests itself to Bloom as a good time for a late-summer getaway from the city.