Thanks for the mention @8BitCrab. Like 8bitcrab I don’t have an exact date, but historically the alphas have been on a six week cycle. The first three weeks are done internally, with no public releases. Major new features are worked on at this time. The next three weeks are when builds are on the Steam latest branch (develop), with the release builds happening at the end of the 3 weeks.
Based on that cycle, one can estimate that A19 will go to the main Steam branch around November 4. Again, this is approximate, as the team will only release when they are satisfied that it is as bug free as they will get for that alpha. Also, the team has said in the past they try to avoid Friday releases as they can’t fix things over the weekend - so keep that in mind as well.