The service will take place in Glasgow's St Andrew's Cathedral, a short distance from the scene of the crash at the Clutha bar.
PC Nelis, 36, was one of three people in the helicopter who died; the others were PC Tony Collins and civilian pilot David Traill.
Six people inside the bar on 29 November were also killed.
They were Gary Arthur, Samuel McGhee, Colin Gibson, Robert Jenkins, John McGarrigle and Mark O'Prey.
Although the area around the cathedral will not be closed for the service, police have warned that traffic may be stopped briefly on Clyde Street to allow for the arrival and departure of PC Nelis' family.
The Air Accident Investigation Branch (AAIB) have started an investigation into the crash.
An initial report into the disaster said the AAIB had found no evidence of a “major” engine or gearbox failure in the police helicopter.