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.