Wedding rings were worn on the left hand due to a Roman myth about a vein

The tradition of wearing a wedding ring on the fourth finger of the left hand derives from the Roman concept of the 'vena amoris' — the 'vein of love' — believed to run directly from that finger to the heart. The anatomical claim is false: no such dedicated vein exists, and all fingers have comparable vascular connections. The Romans nevertheless adopted the left fourth finger for betrothal rings, a convention passed through medieval European culture into modern Western practice. In many Eastern European and South American cultures, the wedding ring is worn on the right hand instead.

Source: Jones, Rings of Desire: Erotic Innocence and the Victorian Wedding Ring (2000)