http://www.xcracer.com/UK-Mountain-Bike-Event-Diary-Planner.php looks pretty comprehensive, as does http://www.sleepmonsters.com/v2_calendar.php

Is there one site that lists everything ?

comprehensive but missing all the gravity events
https://www.rootsandrain.com/calendar/#!/
https://www.britishcycling.org.uk/events?search_type=upcomingevents&keywords=&resultsperpage=30&series_id=&fromdate=18%2F02%2F2014&todate=&zuv_bc_event_filter_id%5B%5D=7&series_only=0&online_entry_only=0&distance=&postcode=

worldwide enduro calendar

http://enduromtb.tv/en/calendario-2/

Roots and rain is bloody brilliant. Learnt that my mate had been on average all season 2.4%slower than his mate all 13 season.

I’ll put in a plug in for our site:

Admittedly a primarily Southeast Asia focus, but starting to get more events from UK and Australia

Marathon MTB is great, especially for overseas events.
Australia based, so that is the focus but has most european events covered. Not good for domestic events. XC racer is best there.