I thought I heard somewhere that this Friday's test was just a small one and there was a larger one to follow...that leads me to believe this weekend. Hopefully at least :D
I think the value lies in the fact they do not want to make promises they cannot keep. I am sure they have several additions or changes they want live for each new test. As long as the changes don't bork the game, no problem.
But, as any of us that have written any amount of code can tell you, complex code can get borked pretty easily. So, they say "Test tomorrow! Yay!" and we are all "Yay! We love Tiny Speck!" and then they realize someone coded some tiny little thing wrong, and now vendors can't sell correctly and the game is so slow to load as to be unplayable. "Now they have to say "OK, so... ummmm... that test we promised for today? Well, there is no easy way to say this... so... ya, that isn't going to happen."
Then things get ugly because players(testers) are a fickle lot. So now everyone is like "Boo! The devs are just toying with us! Liars! Down with Tiny Speck!" So really, it is our own fault.
Orios has it right. In a perfect world we'd be able to announce a test schedule and stick to it, but often it just doesn't work out like that. We get ambitious with the features we want to release, things take longer than we expect, and sometimes we have servers blow up or storage systems go 'wonky'. All of this results in a somewhat inconsistent testing schedule. But, we always announce the start and end of tests here: twitter.com/isglitchopen.