An invitation can simply inform about when and where an event is going to be, but it can be a lot more. It might be illustrative for the event itself, may want to attract people, may want to give an idea for the atmosphere of the event, and it will inevitably -want it or not- give an image... of the sender!
Do not leave the invitation creation on the last-minute. The time for sending the invitation is one of the key elements for the success of an event, especially when addressed to recipients that are not "your people".
Invitations are among the most sensitive things in their relationship with time. They require not only good design but also timely use. They should not be sent too early but certainly not too late!