Many would say release as soon as it is ready! Unless, of course, it’s a seasonal app, like Christmas, Easter, and so on.
Another idea is to release your app during a big hardware launch (new iPhone, iPad, etc). This also has its ups and downs and might not always prove to be the best idea.
– On one hand, during a hardware launch there’s a lot of new devices coming out and new owners are exploring the app store for the first time, increasing the probability of noticing and downloading your app.
– On the other hand, releasing a small independent app during a time when there are a lot of big name app sales, probably wouldn’t get the expected coverage.
As of release days, app downloads peak on Saturdays and Sundays. If you were to release your app on Monday, you might miss out on the weekend boost.
When it comes to avoiding certain days:
– If your app is meant for the Android Market, then stay away of Thursdays as only 7% of apps reached the top 240.*
– On the Apple App Store, the worst day for a new release is Friday, with only 10% of apps reaching the top 240.*
* study conducted by Mobilewalla