Which are the best days to announce startup public launch?

If your promotional materials are ready, I think any day except for the weekend would be a good day.

If we're talking specifically about Product Hunt launches, weekends tend to be low traffic, so probably easier to get awards but fewer eye-balls on the actual product. It also depends a bit on your industry and competitors, def. don't want to accidentally launch on the same day as a competitor or even another big launch that would take away from your buzz