The phrase simply means to get married. 'They're tying the knot in June' is a cheerful, informal way of saying a couple is getting married. It is one of the most common informal expressions for marriage in English, alongside 'get hitched,' 'walk down the aisle,' and 'say I do.'
The most widely cited origin is the ancient Celtic practice of handfasting — a pre-Christian betrothal or marriage ceremony in which the hands of the couple were literally bound together with ribbon, cord, or cloth. The tied cord symbolised the union. Handfasting was practised across Northern Europe and the British Isles for centuries. The literal tying of a knot between two people made the metaphor direct and lasting.
The idea of a knot as a symbol of binding is ancient and cross-cultural. Hindu wedding ceremonies include a literal tying of garments. Roman brides wore a 'knot of Hercules' in their girdle, which the groom symbolically untied. The knotted thread appears in numerous marriage traditions across Asia, the Middle East, and Africa. The English phrase specifically derives from the Celtic and northern European tradition but reflects a near-universal human symbol.
Yes. Handfasting has been revived as part of neo-pagan, Celtic, and Wiccan wedding ceremonies. It is also sometimes included in secular humanist ceremonies as a symbolic gesture alongside or within a civil marriage. In Scotland, historical handfasting was a legally recognised form of trial marriage for a year and a day.
Yes — 'tying yourself in knots' means making something unnecessarily complicated, and 'cutting the knot' (or 'untying the Gordian knot') means solving a complex problem decisively. Knots as metaphors for connection and complexity appear throughout English idiom.