The slogan of this year's rally will be "We ask for a reform of the pension reform, as we can't accept destitution in the old age".
The federations have not yet agreed in detail on the event's programme.
A march of unionists will start in the central Ban Jelacic Square at 1000 hrs Tuesday towards the park.
The costs of the Maksimir gathering amounting to 250,000 kuna will be covered by the city government.