More than 9000 nurses today start a 12-hour strike over pay and staffing levels.
This is a second wave of protests over pay and staffing levels.
More than 2,000 appointments and procedures have been cancelled, including a number of elective caesarean operations.
Information can be found on the health trusts' websites.
The Health and Social Care Board said it expects "significant disruption".
Some schools for children with special needs will be closed as they cannot provide medical cover.