Formal
Formal  events have rules. 'Formal clothes' or 'formal attire' is a way of dressing to meet the rules for an event, such as a party or wedding. 'Formal attire' can be a suit, a tuxedo or a dress. People from all over the world have their own ideas about 'formal attire'. Most of the time formal attire clothes are the most beautiful or the best clothes a person owns. The opposite of "formal" is informal.  Informal clothes are comfortable clothes for general use, e.g. T shirt and jeans.  An informal ceremony might be a ceremony which is very relaxed instead of following an exact plan.  An informal way to greet a friend might be: "Hi, John!"