Just a placeholder right now.  IF anyone gets word on if it is Historic or Seasonal, please post here. 

edit to add. Thanks phearal for the clue to Master Season.

So, I don't really play AL as most of the places around me charge, and when I did go was very....well it left a lot to be desired, but I digress.

For candlekeep, I don't imagine it being an AL run module, simply because they're technically one-shots. The book isn't outlined to be run AS a module, but just one shots. You can run it as a module though...technically since it goes from lvl 1-16. But it doesn't state, from what I've read, the XP people get, or if they level at the end of each one-shot mystery adventure. 

WotC updated the Masters content catalogue today, and included a section for Castlekeep.  I don't think they're viewing this anthology as part of any specific season.

The connection to Candlekeep is reoccurring theme in the Masters campaign and so the D&D hardcover Candlekeep Mysteries is legal for both existing characters in the Masters campaign and for brand new 1st-level characters created specifically for the Masters campaign using the rules described below. All rules in the Masters Campaign Player’s Guide apply to these characters except where superseded by rules in this section.

Quote from Inumiru >>

 I don't imagine it being an AL run module, simply because they're technically one-shots.

...so like pretty much all the adventures...  my 1.5 cents says they'll be AL but not a season on its own.

It is Master's Content only. This section doesn't make any mention of Seasonal or Historic, and it would.  CM "is legal for both existing characters in the Masters campaign and for brand new 1st-level characters created specifically for the Masters campaign".

Quote from Phearal >>

WotC updated the Masters content catalogue today, and included a section for Castlekeep.  I don't think they're viewing this anthology as part of any specific season.

The connection to Candlekeep is reoccurring theme in the Masters campaign and so the D&D hardcover Candlekeep Mysteries is legal for both existing characters in the Masters campaign and for brand new 1st-level characters created specifically for the Masters campaign using the rules described below. All rules in the Masters Campaign Player’s Guide apply to these characters except where superseded by rules in this section.