Platform: OS X Server
Introduction and announcement; not a sales date.
Apple source ↗On June 11, 2012, Apple announced July availability for OS X Server at US $19.99, requiring Mountain Lion. Its July 25 announcement confirmed Mac App Store availability. Server and the Mountain Lion client had the same US $19.99 price but were separate products: buying Server did not replace the underlying operating system.

Apple · Server / Mountain Lion · 2012-06-11 · Apple · Server / Mountain Lion · 2012-07-25
Apple's June 2012 overview described Server as an application added to Mountain Lion. It included file sharing, Wiki Server, Profile Manager for configuring Macs and iOS devices, Time Machine backups, mail and calendars. The technical specifications require Mountain Lion, 2 GB memory and 10 GB free, with more space for some features. That 10 GB is not the client's stated 8 GB requirement.
Apple · Mountain Lion Server · Technical Specifications · Apple · OS X Server · June 2012 · pp. 4, 25
Application and operating-system numbering diverged: OS X Server 2.x here belongs to Mountain Lion, not a nonexistent OS X 2.x. In January 2013, Andrew Cunningham explained in Ars Technica that Server and the operating system could update separately. He also described DHCP returning to Server app and more Time Machine backup information; these were later changes, not established launch-day features.
Andrew Cunningham · Ars Technica · 2013-01-16
In The Register in August 2012, Keith Martin praised the clarity of mail administration and server-managed Time Machine backups. His test also described trouble when returning a test Mac to normal use with its local DNS still configured. This is attributed experience, not a universal fault. The preserved technical specifications include Caching Server, but their entire list is not projected back onto the original launch without distinguishing updates.
Apple · Mountain Lion Server · Technical Specifications · Keith Martin · The Register · 2012-08-30 · p. 2
Selected releases, not a complete history. Dates follow Apple's release records, not the latest page edit. No builds are included.
ISO date: year-month-day.
Platform: OS X Server
Introduction and announcement; not a sales date.
Apple source ↗Platform: OS X Server
Mac App Store availability confirmed by Apple; requires the underlying OS.
Apple source ↗