The decision was taken after a weather warning was issued of strong winds and heavy rain.
With no weather warnings in place for this evening, tonight’s event will go ahead as planned from 6pm – 9pm.
Head of Culture with Council, Aeidin McCarter, said while disappointing, health and safety was the main priority in making the decision.
“We have been monitoring the weather situation throughout the week and have made the decision to cancel the outdoor element of tomorrow’s event.
“This is always a risk when staging any outdoor event and while this is very disappointing – particularly for our events team and performers who have put so much effort into planning and creating this experience – health and safety is always the main concern.
“Thankfully the forecast is fine for today and we can go ahead with the Trail events as planned, so we look forward to welcoming visitors this evening.
“And plans for Friday’s night’s main Halloween celebrations are still in place.
“With more people expected to attend this evening we would advise that you check the traffic and travel information, and be mindful that large crowds are expected which may put pressure on parking. Plan your journey in advance and use public transport if possible.
“We apologise for any disappointment and promise a spectacular show this evening and Friday.”
Indoor events in the Guildhall and other venues will go ahead as planned on Thursday, with children’s shows and activities scheduled throughout the day, as well as a live performance by popular local band Ports at 9.30pm under the Samhain Moon.