I wish I could be more exact, but I don't think we've announced the exact date. However, you shouldn't have to wait long. 
When 8.5.3 Upgrade Pack 1 is released, it will be available in two places:
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Customers on active maintenance can download from Passport Advantage.
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IBM Business Partners can download from Partnerworld Software Access Catalog.
Those bullets are from the tech talk slides Bruce mentioned.
-- Dave