What's considered the best party time for your celebration? Your child's actual birthday may not be the best time, so a weekend might pose a better prospect. I'd probably go for Sunday. If you work during the week, this'll give you Saturday for last-minute shopping and preparations and Sunday morning to setup. As for the time of day, it depends on the age. For little ones, 10:00-Noon is a good party time because that way you work around nap time. For the older children, it really depends on what their schedule is like and what is customary on your end of the world. If you live in a country where folks religiously Siesta, you won't find ANYONE awake during the siesta hours.
However, I've often heard that for non-"Siestaers", the best party times are 1:00 - 3:00 or 2:00 - 4:00. During these hours, you may consider it an added advantage that you won't have to serve a meal. Although, personally, if I'm already making the effort, I'd rather go all the way and make creative foods that fit with the party theme (as I've detailed in a previous blog titled "French Flies and Toad Slaw." - Feb. 24th, 2005)
How long should the party be? One and a half hours should be quite enough time for a toddler or pre-school party. For older children two to three hours should be fine.