larunbat.

Two possibilities exist: The first etymology argues that the word would be composed of laurden ("quarter") + bat ("one"). According to this theory it is unclear if the word would have first designated a quarter of the month, and then became the name of a day in the week (because a week is roughly a quarter of a month), or if it was already a day by itself. Some also argue that the word would mean fourth day, the etymology thus being laugarren ("fouth") + egun ("day"). But these etymologies contain multiple flaws. First it would suppose that in ancient times basques would have used a three-day week (that would evolved towards a four-day week with larunbat), a theory that is not generally accepted by linguists (see astearte "Tuesday" for more details). Moreover the etymology faces anachronism given that it uses the modern genitive laugarren, whereas medieval Basque didn't use the -r- for genitive but -e-, the form being lauen. Seeing those flaws, Koldo Mitxelena proposes a second, and the dominant etymology : deriving the term from lagunen bate (a verbal noun form of batu) meaning "gathering of companions" on the model of neskenegun/neskanegun ("Saturday"). He proposes the evolution of the word as the following : lagunen > *laguren > *laurun > larun, with dissimilation of the nasal consonant n into r and loss of /e/ in bate.