I've added the release dates for Plasma 5.12, .13 and .14 as well as Frameworks releases to the end of the year onto the calendar on Phab.
https://phabricator.kde.org/calendar/ Which makes more sense than the Google calendar I had on my personal account. You should be able to export the .ics and import it into whatever calendar you like to use. Jonathan Not the boss, not the chief and not secretly controlling the whole KDE release process