The best day to launch your app is either Wednesday night or Thursday, because the iOS App Store changes features every Thursday and you may get featured that day as "New games..." "New app..." etc. Ideally all your marketing/PR/advertising efforts should start that day (Thursday) because Apple likes it when an app is being privately promoted at the same time it's being featured.
Yes AdMob could work for you but please know that there are many other ad placement companies similar to AdMob that can work for you too. You need to see what kind of clients they have, if they match with your niche, etc. Usually this is a trial and error process, you start with several ads and you see which works and which doesn't, you improve, you cut them. etc. Good luck