(mine was a weekly event without end)   

I found a solution and want to share it here if someone has the same issue one day. I managed to manipulate the calendar database and deleted the item using SQL. * Download a sqlite-client, I used the demo of razorsql. * Close TB to release database usage. * Open a connection to TBs database file (take a backup of the file before, just in case), it is located under: /home/<user>/.thunderbird/<profile-name>/calendar-data/local.sqlite * Browse the cal_events (in case of an event) table to locate your event by name and determine the ID (foreign key name in other tables: item_id) of it. * Delete the event in all tables you found it (by item_id), like so: (mine was a weekly event without end) delete from cal_recurrence where item_id = '1e2b7f24-10fc-4b47-8d4a-66a50b4f920a'; delete from cal_alarms where item_id = '1e2b7f24-10fc-4b47-8d4a-66a50b4f920a'; delete from cal_properties where item_id = '1e2b7f24-10fc-4b47-8d4a-66a50b4f920a'; delete from cal_events where id = '1e2b7f24-10fc-4b47-8d4a-66a50b4f920a'; * Add the event again if you need it. Voilà.