The three most essential components of any barbecue party are the guests, the weather and the food. I can not make suggestions about your friends and family, you are on your own there, but I hope you will find my suggestions for the other two fronts practical.
An impromptu party in the garden is great and often the best parties are the ones that just happen off the cuff, but if you want something a little more formal or more impressive then you have to organize. The first step is to pick a date far enough in the future for most people to be unlikely to have a previous engagement, but not so far that they might forget about your barbecue party.
Depending on where you live of course, try to pick a date when it is unlikely to be cool or wet. This is hard in places like the UK, but may be simpler where you live. If the party goes on into the night it may get cool anyway and you could hire a few gas patio heaters in advance just in case. You could also have a sunshade or sheltered seating area in case of blazing sunshine or a light shower.