In a spectacularly erudite line of reasoning Nathan Halper, in Twelve and a Tilly: Essays on the occasion of the 25th anniversary of Finnegans Wake (Faber and Faber, 1966), proves that the night on which Finnegans Wake is set, or rather the night during which the dream of Humphrey Chimpden Earwicker takes place, must be that of Saturday March 18th 1922 (the year in which Ulysses was published).
I can see this catching on.
As to what happens before stately, plump Buck Mulligan begins his morning ablutions on June 16th, we know from Ulysses itself what happened the previous night, at least in the Martello tower in Sandycove. There were alarms and excursions, and shrieking nightmares for a visiting Englishman from Oxford named Haines, an incident which in real-life led to Joyce's abrupt departure from the temporary address that would become part of literary history.
Joyce based Haines on a chap called Dermot Chenevix Trench. An illuminating article about him by one C. E. F. Trench appeared in James Joyce Quarterly Vol. 13, No. 1 (Fall, 1975) and can be read here..
More Joyce tomorrow, of course.