From an agricultural viewpoint, this is the Festival of the First Fruits. The crops have long since been planted and are well along in their growth, but are not fully matured. The hard work of planting is in the past and the hard work of bringing in the harvest is in the future. This is a time to kick back and relax.

In Ireland, this is the festival of Lughnassadh (Lugh = the deity + násad = assembly; festive or commemorative gathering¹).

Lugh is one of the Gods of ancient Ireland. His father was Cian, of the Tuatha Dé Danann (the Gods). His mother was Ethniu, the daughter of Balor, the king of the Fomoire (the Giants). His foster mother was Tailtiu, who taught him all the arts of a warrior and who later died from the exertion of clearing a great plain in Ireland for agriculture³. Lugh is known as Lugh Samildánach - "Skilled in All the Arts" - because he is a master of all crafts, and Lugh Lámhfhada - "Lugh of the Long Arm" - as the wielder of the Spear of Victory, who slew Balor with his sling in the Second Battle of Maige Tuired, thereby leading the Tuatha Dé Danann to victory over the Fomoire^{1, 2}.

Traditionally, Lugh founded this festival to honor his foster mother Tailtiu ^{2, 3}, although Miranda Green mentions that it might commemorate Lugh's wedding⁴. Lugh himself led horse racing and contests of martial prowess. The early Irish continued this festival by holding a large fair in what is now Teltown in County Meath, and similar fairs in other provinces. People came from far away to attend this fair. It was considered to be an ideal time to get married, either permanently, or in a trial marriage lasting a year and a day.³.

Thus, this festival is traditionally a time of fairs, community gatherings, and family reunions. It is an occasion of truce, featuring warrior games of strength and skill and is a favored time for marriage.

In published ADF Lughnassadh rites, Lugh is always the main Deity of the Occasion. We celebrate the wedding of the god of light with the goddess of the land. We offer loaves of bread and beer, and play games.

¹ —. Cath Maige Tuired. Ed. and tr. Elizabeth A. Gray. Kilare: Irish Texts Society, 1982. Stanzas 53-71 tell of the arrival of Lugh Samildánach at the court in Tara. Stanzas 133-135 tell of Balor's defeat at the hands of Lugh.
² —. Lebor Gabála Érenn. Ed. and tr. R.A. Stewart Macalister. 5 vols. Dublin: Irish Texts Society,1938-1956. V.4 speaks of the Tuatha Dé Danann. p. 107 says they brought "the spear which Lug had : battle would never go against him who had it in hand" from their city Goiras. p. 179 says that Tailltiu's "games" were celebrated every year for a fortnight before and a fortnight after Samain (read Lugnasad, says the editor).
³ James MacKillop. Dictionary of Celtic Mythology. Oxford: Oxford University Press, 1998. p. 276 Lughnasa and page 348 Tailtiu.
⁴ Miranda Green. Dictionary of Celtic Myth and Legend. New York: Thames and Hudson, 1996. p. 136 Lughnassad suggests that it might also commemorate the wedding of Lugh.

I should find Máire MacNeill's The Festival of Lughnasa, Oxford 1962. This seems to be a definitive secondary source.

Other than half a sentence in Miranda Green, I found nothing to indicate that this festival is a "wedding feast". I found no basis for Ian Corrigan's translation of Lughnasadh as "the wedding feast of Lugh". I found nothing to explain this festival as "the wedding of the god of light and the goddess of the land", as published rituals assume.