But it doesn't say when they will announce that. They could wait two more months (or any other period of time) then say "We'll tell you next Tuesday."
It does:
Go to Dream Theater’s Facebook page (https://www.facebook.com/pages/Dream-Theater/7677942180) on Thursday, 4/21 at 11 AM to sign up for the mailing list and to receive more details about an exclusive and intimate look at the auditions, and to find out exactly when and how the momentous announcement about the band’s new lineup will be made.
Thursday at 11 AM they will say exactly when and how the "real" announcement will be.
That is not how I interpret that sentence. To me that says, 'You can sign up for the mailing list Thursday.' It does not say 'We will be sending things out to the people on the mailing list on Thursday.'
Maybe it's just me, but it seems pretty clear what is going to happen. On thursday, on their facebook page, you will be able to sign up for a mailing list. On thursday, information will be provided on their facebook page regarding the "exclusive and intimate look at the auditions". On thursday, you will learn when you'll know the new drummer, and how.
You will not know the new drummer on thursday. This much is obvious from the word "when" in the last sentence ("..to find out exactly when and how the momentous announcement will be made")... Unless you consider this a possible Thursday message: "you found out when the announcement will be made: just a second ago, when you read the sentence telling you who it was! congrats!".
You probably need to sign up for the mailing list to get further info about the "exclusive intimate look at the auditions" (to actually get the exclusive look, rather than to be told how the exclusive look will be transmitted, which is what is happening Thursday). Bosk may or may not let this info leak onto DTF without you signing up for the mailing list, but it'll be on the internet somewhere anyway, I'm sure.