Timetables generally come out in December for the following summer.
The 2020 Leaving Cert Timetable can be viewed here: https://theleavingcert.com/leaving-cert-2020-timetable/
Very rarely does the timetable change once published. However, in exceptional circumstances it has happened in the past – for example in 2009 an English Paper 2 exam had to be rescheduled after a superintendent issued Paper 2 to students who were sitting a Paper 1 exam. The news leaked online and in the media and therefore the Paper 2 exam had to be pushed back to a Saturday in order to figure out the logistics of distributing new papers quickly to all exam centers.