Wikimedia groups like WMES don't need to install their own Phabricator instance. They can use Wikimedia Phabricator instead.

Only the users willing to introduce and manage calendar data would need to create an account here (which they can do with their Wikimedia accounts). Users only needing to access that data can visit the URLs of the calendars they are interested about, and according to this proposal they will be able to import Wikimedia Phabricator calendar data to their own calendars importing the iCalendar files (.ics)

Users with a Phabricator account will be able to RSVP, comment, and associate events to other projects (promoting activities to the users following those projects).

@BrillLyle, we are not trying to compete with Google Calendar, but I think we can aim to provide the basic functionality that most people may need for Wikimedia activities. From your list:

* Time zone support. Events are created with the timezone of the event creator, and that [[ https://phabricator.wikimedia.org/settings/panel/datetime/ | timezone can be defined ]] in the user profile. As of today, events cannot be created in a different timezone, but this is a problem with a simple workaround (find out the time in your own timezone). Not a blocker.

* Color coding. Missing indeed. Icons can be defined for events, and if this is considered important then perhaps colors could be defined as well, just like it is possible to define icons and color for projects. This looks like an improvement simple to implement that would provide basic color cues in calendars.

* Ability to select and maintain different calendars. This is possible, by associating events to projects (i.e. ArchCom meetings, Tech Talks, Wikimania Hackathon 2016...) and by [[ https://phabricator.wikimedia.org/calendar/query/advanced/ | querying events ]] in a pretty advanced way (i.e. IRC Office Hours about VisualEditor during 2015, displayed in month view). These queries provide static URLs that can be shared with users.

* Ability to copy to events to personal calendar. Phabricator users can RSVP already now, and this will make events appear in their personal Phabricator calendar. According to this proposal, they will be able to import the .ics data as well, which will integrate such data in their calendar apps.