What is the best day for releasing a project, and what time?
Goblin would choose a day when you can stick around after launch to catch broken downloads or answer questions. Then pick an hour that suits the time zone of the people you plan to tell about it.
If you don't have an audience yet, treat the first release time as a starting point to learn from. Check the page and download while signed out before sending your announcement. Goblin wouldn't promise that a particular weekday or hour will bring more players.
There is no magic day, but a few things are worth avoiding: releasing during a big event like Steam Next Fest, a major sale or the end of a popular jam means your announcement gets buried in a flood of other posts. Many people pick Tuesday to Thursday so they have the following days to fix bugs before the weekend. For the hour, aiming for late morning US Eastern time is a common compromise because it is also afternoon in Europe. The most important part is being available for the first 24 hours to answer comments and push a quick fix if something breaks.