I think that sounds great...especially if you do it on a nice summer's night! I would say do it on a Friday or Saturday though. I threw one for my friend and she wanted it on a Thursday night and she lived quite far from everyone so not too many ppl actually showed! But if it's like that on a weekend I'm sure it'd be different!
What would be awesome is to plan it on a full moon if you can. I just looked at the calendar and the only weekend that has a full moon is June 26th. But you could always plan one that's right before so the moon is nice and bright for it!! :)
It shouldn't make a difference, just make sure it's not a Sunday night when many people go to work the next day. Also, if many people are traveling much it might make it harder for them to come to the shower unless they get a hotel room, keep that in mind too.
It does sound like a cool theme! I can't wait for everyone to start having there showers and share how they all went!
I doubt that day or night would really make a difference. My sister had her baby shower at night at a resturaunt so that people could drink and party at the same time I suppose. It ended up having a HUGE turn out. Your theme is so cute that I say do it at night. If anything people will think its fun and creative! I do! :)