Multiple realities
(covers information from several alternate timelines)
A wedding ball or wedding reception was a dance event given to celebrate a wedding.
In 2268, Sylvia suggested to "William Claiborne" (Pavel Chekov) that they change the dance which was a week hence to a wedding ball instead (i.e. that they marry). (TOS: "Spectre of the Gun")
While traveling in time, Kes was told by Tom Paris about their wedding reception and that Harry Kim had been their best man. (VOY: "Before and After")