If you look at most professionally designed fonts, you'll notice that many use two fonts: one simple and easy-to-read typeface in which crucial information like the time and place is set and one decorative font that is used emphasize certain information. For instance, on many birthday party invitations, you'll see words like 'Where' and 'When' set in a bold typeface that grabs attention, letting a reader quickly focus on the information next to it in a simpler font.
Pairing fonts for your invitation may be made simple by the fact that you already have one font in mind. A decorative font may have caught your eye — or you may have a specific style to follow, like a font matching the logo of a large event — and you just need to find a simple font to go along with it. In such cases, it's important to find a font that can be easily read and has at least a general feel that matches your decorative font. You wouldn't want to pair a cursive font, for instance, with a block letter text font. You'd want something with a little grace.