Platform: Mac; Apple announcement
Announcement and developer preview, not public availability.
Apple source ↗OS X Yosemite 10.10 was announced on June 2, 2014 with a developer preview; the public system reached the Mac App Store free on October 16. Its design updated icons and typography and introduced translucent elements. Notification Center's Today view added widgets, while Spotlight moved to a central presentation with new result categories.

Apple · Yosemite · 2014-06-02 · Apple · Yosemite · 2014-10-16
iCloud Drive appeared in Finder to organize cloud files. Mail gained Markup for annotating images and PDFs and Mail Drop for attachments up to 5 GB. Safari changed its interface and Messages expanded group conversations. OS X Server 4.0 was a separate US$19.99 purchase, not part of the free client download.
Apple · Yosemite · 2014-06-02 · Apple · Yosemite · 2014-10-16
Continuity connected Mac and iOS through Handoff, Instant Hotspot, iPhone calls and SMS. Not every Mac or feature had the same requirements: specifications distinguish Handoff and calling with iOS 8 from Instant Hotspot and SMS with iOS 8.1. The latter system was announced for October 20, after Yosemite's launch; some features needed newer Macs and carrier service.
Apple · Yosemite · 2014-10-16 · Apple · Yosemite · Technical specifications
General minimums were OS X 10.6.8, 2 GB of memory and 8 GB free; Apple supported Macs introduced from 2009 and some 2007 and 2008 models. Photos arrived in 10.10.3, not the initial system. The security archive dates that update to April 8, 2015 and 10.10.5 to August 13, 2015. These selected dates do not enumerate every point release.
Apple · Yosemite · 2014-10-16 · Apple · Yosemite · Technical specifications · Apple · Security archive · 2015 · Apple · Photos / 10.10.3
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: Mac; Apple announcement
Announcement and developer preview, not public availability.
Apple source ↗Platform: Mac OS X client
Available to download that day, according to Apple.
Apple source ↗Platform: Mac OS X client
Release date recorded by Apple; client update.
Apple source ↗Platform: Mac OS X client
Release date recorded by Apple; client update.
Apple source ↗