A convention is a meeting, usually of a particular group. Political parties, teachers, plumbers, gardeners, toymakers and computer designers all hold conventions.
In fact, lots of cities have built Convention Centers in hopes of attracting convention-goers. The best-known conventions happen every four years when the Democrats and Republicans meet to nominate presidential candidates. A convention can also be used to describe the normal or accepted way of doing things. It's the convention, for example, for your employer to give you a three-day weekend around the Fourth of July, even if it falls on a weekend.