The banns of marriage or, simply "the banns", (from an Old English word meaning "to summon") are the public announcement from the pulpit that a marriage is going to take place in that church between two specified persons at a specified time.
Their purpose is to allow anyone to come to the wedding to raise any legal impediment to it—such an impediment might be a prior marriage (or pre-contract or betrothal, those being legally the same as a marriage), or a vow of celibacy, or the couple's being related within the prohibited degree of kinship, or lack of consent—to prevent marriages that are legally invalid, either under canon law or under civil law.
In England, under the provisions of Lord Hardwicke's Marriage Act, the reading of the banns became necessary for the validity of a marriage. By this 1753 statute, 26 Geo. II, c. xxxviij, the banns had to be read aloud in church over a period of three Sundays prior to the actual wedding ceremony. Omission of this formality rendered the marriage void. Prior to this law, it was possible for eloping couples to marry clandestinely in various places—finding an imprisoned clergyman in the Fleet Prison was one well known way (a "Fleet Marriage"), at least for couples near London. After the law, elopers had to leave England, usually for Scotland, and proverbially, to the village of Gretna Green, in order to contract a marriage while avoiding these formalities. These details often figure in melodramatic literature set in the period.
In Ontario, the publication of banns remains a legal alternative to seeking a marriage license. A same-sex couple attempted to marry this way at the Metropolitan Community Church of Toronto in 1999, since the province was not then issuing marriage licences to same-sex couples. The marriage was ruled valid in 2003. See Same-sex marriage in Canada.