Rather than hosting one four-hour event on 5 July, Le Petit Viaer Marchi will be held instead at the Folk & Costume Museum across 17 and 18 July.
Crowds of up to 7,000 would usually bring much-needed revenue in for the trust to maintain historical properties, artefacts and land.
‘It is our biggest individual fundraiser, no doubt about that,’ said National Trust president Tony Spruce.
The losses should be roughly balanced by lower set-up costs of the smaller event.
‘Costs to set up will be minimal here, so we should take a reasonable amount through the sale of tickets and manage OK.’
Hosting across a weekend also gives islanders more flexibility to attend.
Sufficient notice for caterers, musicians, smallholders, food vans and other suppliers was another reason to downsize.
‘Our caterers have to know well in advance to prepare 800 gallons of beanjar, which goes to waste if we cancel late.’
All restrictions on gathering size are set to be removed from 30 April in the Bailiwick blueprint.
Given borders are due to reopen completely on 1 July, hosting the full-scale event four days later was considered a possible super-spreading event risk by the organisers.
‘If we get in a situation where we end up having to cancel last minute, that would be a mega loss for us.’
With Liberation Day celebrations in Town curtailed, it was decided to proceed cautiously.
‘If Covid was thoroughly dead and buried we would, but we will not take the risk.’
Around four to five hundred could still attend at once.
‘We would not be able to manage crowds and sign them in with a full-blown Viaer Marchi, but we could here.’
Funds are sourced from trust memberships, bequests, property rentals, corporate events and the Fermain Tower.
‘Fermain Tower is highly booked for the season,’ Mr Spruce said.
‘I won’t say fully because there are still some spaces.’
Work to the triangular 1850s St Sampson’s property, Church House, is well under way too, Mr Spruce said, which it is hoped will become a Guernsey granite industry museum.
‘We’re hoping to get it done this year. Surprisingly, for a house that old, there was hardly any rot in the roof when we took the slates off.’
All Folk & Costume Museum exhibitions are included in the Le Petit Viaer Marchi entry fee.