When it is ready. Or when it needs to be released (like when new devices are available to the market).
Apple is not a company that normally announce release dates in advance, it's not how they operate.
Without speculating on the final release date, you may count on it being a number of beta releases during the summer and the (early) autumn and each beta release tend to have one or a couple of weeks between them.
Khedron
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Thank you @CDUVENHORST, of course the problem is that my old MBP is too old. Didn't checked that until you mentioned it.
Have the same problem as atineoSE, I only see the 10.5.6 beta.