Juvederm is considered a "lunchtime treatment" , meaning that you can go back to work after the treatment. However, anytime you get an injection there is always a chance of getting a bruise. Usually this can be covered with makeup however, rarely, the bruise can be severe enough to be a cosmetic issue.
The best way to avoid a bruise is to discontinue any medications that thin the blood for one to two weeks. The most common are aspirin, NSAID's (Motrin, Advil, etc) and Vit E. If your doctor has recommended these medications, always check first to be sure it is OK to stop.
Juvederm can also cause some swelling for the first week or two.
Finally you may not be totally satisfied with the "look" of your treatment. The good part about Juvederm is that it can be quickly reversed. However you would want to wait a few weeks before dissolving the treatment because, if there is swelling, things may look great after a few weeks and it would be a shame to waste the treatment.
I would have the treatment at a time when there is a few weeks before a big event. Most likely, it won't be necessary, but that is the safest way to go.