Around 1,000 surgical operations are likely to be cancelled due to a public sector strike this week.
Health and education workers in the Unison union are planning strike action tomorrow against cuts and job losses.
Unison general secretary Patricia McKeown said an agreement has been reached over emergency ambulance cover but "widespread disruption" is expected for patients due to have elective surgery, as well as outpatient clinic appointments.
Picket lines are set to begin forming shortly after midnight at the Mater Hospital in Belfast with lines forming at hospitals across Northern from early tomorrow morning.
Ms McKeown said some schools will close and social services would also be affected.
The Northern Health Trust has set up a patient information line for between 2-8pm today and 8am-6pm tomorrow at 028 2563 3758.