If you have never used a professional event photographer, here are some factors you should be aware of and consider:
The Photographer: Make sure your photographer knows how to shoot events similar to yours. Beware of photographers with lots of expensive equipment, but no real experience in photographing events. Check out their advertising and website and see if they say anything about event photography; or are they really a portrait, little league, or high school graduation photographer without experience in large events. Ask them what events and celebrations they've worked, remember, the photographer may not share his event photography publiclly, but may have a nice portfolio to share with you privately. Most importantly, be comfortable with your photographer, ask questions, be clear of your expectations, and be open to the photographers creativiity.