The most important information should be clearly visible on the invitation. This includes, of course, the names of the bride and groom, the wedding date, time, and venue(s). Make sure that this information is easily readable for all invited guests (don’t use too small a font!) and stands out from other text elements.
The rest of the invitation text can be formal, funny, or informal, depending on the tone you prefer for your wedding. You can use traditional wording or let your creativity run wild. The important thing is that the text makes guests feel welcome and appreciated.