My first app was launched on a Friday night and it's first full day of launch was Saturday. It got some sales and the free version got about 800 downloads.

What do you think the best day/time to launch is to maximize potential sales while the app is in the "Just In" section?

(I'm planning to launch my second app this Friday evening again unless you have better thoughts about day/time)

I launched Friday too and made three sales. However, it stopped there not because the app I made sucked but because of two crippling bugs. Lack of support for Nexus One trackball (I was using the D-Pad) and lack of support for high resolution screens which in effect rendered the app I wrote in operable and hard to see. So I learned from testing on multiple devices in needed but it was my first time releasing my cell phone app and my excitement got the best of me. Anyways good luck to you.

I live in the UK and I tend to launch my apps at 10pm on a Thursday or Friday night (10pm here is 5pm east coast I think).

However, I have launched apps on different days and times and I have found that the days don't matter much but the timing is quite important - it seems that it works best if it catches the US evening, so 6pm-midnight US time (I have noticed this is when I get most of my sales for my already launched apps too).