Origins
Thursday is Thor's day, and of all the weekday etymologies it is perhaps the most viscerally evocative: the day named for the red-bearded god who rides across the sky in a chariot pulled by goats, wielding the hammer Mjölnir to shatter giants and summon storms. The word's origin connects English speakers to both Roman planetary astronomy and the deepest layers of Germanic mythology.
The word derives from Old English 'þūnresdæg,' a compound of 'þūnres' (genitive of 'þunor,' meaning thunder) and 'dæg' (day). The Old English word 'þunor' served double duty as both the common noun for thunder and the proper name of the thunder-god, Þunor. This is a calque of Latin 'Iovis diēs' (day of Jupiter/Jove), with the Germanic thunder-god substituted for the Roman sky-god through interpretatio germanica.
The equation of Þunor/Thor with Jupiter rather than with Mars or any other deity was natural and consistent across all Germanic-Roman cultural contact. Both were supreme weather gods who commanded thunder and lightning. Both were imagined as physically powerful, bearded figures. Both were protectors of the community — Jupiter as guardian of the Roman state, Thor as defender of Midgard (the human world) against the forces of chaos. The parallel was so obvious that it was made independently by multiple Germanic peoples at different times and places.
Proto-Indo-European Roots
The Proto-Germanic name *Þunraz derives from the PIE root *(s)tenh₂-, meaning 'to thunder' or 'to resound.' This root also produced Latin 'tonāre' (to thunder), from which English gets 'thunder,' 'detonate,' 'astonish' (originally 'to strike with thunder'), and 'stun' (from Old French 'estoner,' ultimately from the same Latin root). The common English word 'thunder' is thus a doublet of 'Thor' — both descend from the same Proto-Germanic *þunraz, one as a common noun and the other as a divine name.
The phonological development from 'þūnresdæg' to 'Thursday' involved a significant sound change. The Old English form contained 'þūnres-' with the cluster /nr/. During the late Old English and early Middle English periods, the influence of Old Norse 'Þórr' (Thor) — brought by Scandinavian settlers in the Danelaw — appears to have reshaped the English form. The Middle English spellings 'Thuresday' and 'Þursdai' show the Norse-influenced 'Thor/Thur-' replacing the native 'Thunres-,' simplifying the word and bringing it closer to the Scandinavian model.
Across the Germanic languages, Thursday preserves the thunder-god transparently. German 'Donnerstag' (from 'Donner,' thunder), Dutch 'donderdag,' Swedish 'torsdag,' Danish 'torsdag,' Norwegian 'torsdag,' and Icelandic 'fimmtudagur' (fifth-day, replacing the pagan name) all point to the same deity. The Germanic forms split into two groups: the West Germanic languages (English, German, Dutch) use forms derived from the common noun *þunraz (thunder), while the North Germanic languages (Swedish, Danish, Norwegian) use forms derived from the proper name Þórr (Thor).
Latin Roots
In the Romance languages, the Latin 'Iovis diēs' produced French 'jeudi,' Spanish 'jueves,' Italian 'giovedì,' and Romanian 'joi' — all preserving Jupiter/Jove. The connection between Jupiter and Thursday survives in English through the adjective 'jovial,' which originally meant 'born under the influence of Jupiter' and thus cheerful and good-humored, since Jupiter was considered an astrologically benign planet.
The cultural significance of Thursday varies across traditions. In Norse mythology, Thursday was naturally Thor's sacred day and was considered auspicious for beginning journeys, signing contracts, and conducting legal business — reflecting Thor's role as a god of oaths and social order as well as storms. In the Christian tradition, Thursday acquired special significance as the day of the Last Supper (Maundy Thursday). In German folk tradition, 'Donnerstag' retained pre-Christian associations well into the modern era, with Thursday thunderstorms interpreted as signs of Thor's lingering presence.
The name Thursday also gave English the surname 'Thurston' (from Old Norse Þórsteinn, 'Thor's stone') and, through Robinson Crusoe, the personal name convention of naming someone for the day they were found — Defoe's character Friday being the most famous example, but the convention extending to all weekdays.