A wedding breakfast is a dinner given to the bride, bridegroom, and guests at the wedding reception. The phrase is still used in British English. The Compact Oxford Dictionary lists the phrase as only “British”, and the Merriam-Webster online dictionary does not list it at all. The custom of the wedding breakfast is occasionally spotted in non-English speaking countries that market themselves as wedding destinations, e.g. Poland.
Nowadays the wedding breakfast is not normally a morning meal, nor does it look like a typical breakfast, so its name can be confusing. The name is claimed to have arisen from the fact that in pre-Reformation times, the wedding service was usually a Eucharistic Mass and that the bride and bridegroom would therefore have been fasting before the wedding in order to be eligible to receive the sacrament of Holy Communion. After the wedding ceremony was complete, the priest would bless and distribute some wine, cakes, and sweetmeats, which were then handed round to the company, including the bride and groom. This distribution of food and drink was therefore a literal “break fast” for the newly married couple, though others in attendance would not necessarily take Communion and therefore would not necessarily have been fasting. Since usage of the phrase cannot be shown to date back earlier than the first half of the nineteenth century however, a pre-16th-century origin seems unlikely.
The author of Party-giving on Every Scale (London, n.d. ) suggests the phrase may have evolved fifty years earlier:
The orthodox "Wedding Breakfast" might more properly be termed a "Wedding Luncheon," as it assumes the character of that meal to a great extent; in any case it bears little relation to the breakfast of that day, although the title of breakfast is still applied to it, out of compliment to tradition. As recently as fifty years ago luncheon was not a recognized meal, even in the wealthiest families, and the marriage feast was modernized into the wedding breakfast, which appellation this entertainment still bears.
The Oxford English Dictionary does not record any occurrences of the phrase "wedding breakfast" before 1850, but it was used at least as far back as 1838. This would agree with the quotation above, which suggests the phrase came into use about the 1830s.