Since Star Parties are mainly outdoor events, you will want to plan the date of your party around the weather. Wait until spring or summer if you live in a cold climate. Bad weather can ruin any outdoor party, so plan ahead and plan well. You'll want your guests to be comfortable, so have plenty of seating available. You may even want to consider opening up part of your home, especially in areas where the weather may flucuate to less than favorable conditions.
You may also want to decide on the date based on astronomical events. You can throw a Star Party at any time, when the goal is simply to view anything from asteroids to planets to star clusters, given its a clear night. But you may want to get more specific and plan it around an upcoming eclipse or meteor shower, for example.
Note: If your goal is to view the night sky at your Star Party then you will want to keep any lighting very low in order to preserve your night vision (preferably monochromatic - red light is best for saving your night vision.)