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

I'm not so sure on the revenue part as I've never done that myself.

But the easiest ways to get started with promotion are:

1. ASO
2. PR. (you can do some of this yourself. Naturally, helps if your app solves a new or interesting problem).
3. SEO (easier if it's a niche app).
4. Paid social ads. I'd start with Twitter and Facebook (though you might want to add inInstagram or Pinterest depending on your app).
5. Recruit plenty of paid product testers now. Get their support/feedback to help build the app. Make them part of the process and then they will probably help spread word of your app on launch day.

Good luck with your app!

If you're hoping for a best time and day to sell your app then I think you're really going to struggle...

What you need is a proven strategy that will create predictable results. Research the top 50 most successful app launches and you will find a pattern, I guarantee it. Distill this pattern into a formula, and then execute.