Ok the calendar is now re-enabled. Unlike the old one, the calendar only appears on the main board index when there is an event to display on that day. You can get to the calendar via the link at the top of the page, next to notifications.
Before anyone asks - I don't know why it thinks everyone's birthday was yesterday. ;D This seems to be a common issue with YaBB board conversions, but I haven't been able to find a solution yet.
Suffice to say, the events list yesterday when i first enabled the calendar was rather lengthy. :s02