ApplicationTransactions is the infrastructure component which creates edit logs, supports comments, and enables a lot of additional features for free or nearly-free. The major components are:

• Create a new PhabricatorCalendarEventTransaction class, similar to classes like AlmanacServiceTransaction.
• Create a storage patch for it, like resources/sql/autopatches/20141025.phriction.1.xaction.sql.
• Create a new PhabricatorCalendarEventTransactionComment class, similar to classes like PhrictionTransactionComment.
• Create a new storage patch for that, like resources/sql/autopatches/20141025.phriction.2.xaction.sql.
• Create a PhabricatorCalendarEventEditor, like other ...Editor classes.
• Convert the "Create Event" workflow to use your new Editor. You probably need to define four transaction types initially: status, start date, end date, description.
• Make the event detail page show the transaction log at the bottom, so you can see, e.g., changes to the event's description.
• (It won't automatically support comments, but we can add those later -- getting this far unblocks everything else which depends on ApplicationTransactions.)

Exciting stuff. :) I hope this includes (or it comes soon after) feed story hook implementations in PhabricatorCalendarEventTransaction...! I think creating calendar events in particular deserves surfacing to the feed.

I made that explicit in T7955, although we probably need to let you name events (T7953) first or the stories are going to be pretty iffy.