Last month, Grabus announced the community had secured the missing funds as Qatar would follow up its earlier donation with a EUR 10.8m grant.
Qatar had donated EUR 15m to kick-start the construction of the mosque and cultural centre, which began in 2015.
The mosque now needs to be installed with the interior equipment, so another EUR 2.4m is needed, which Grabus hopes would be provided by members of the community.
The Slovenian Muslim community head believes the Islamic Cultural Centre in Ljubljana is "a unique project which has left a mark on our generation".
"To build a mosque or plant a tree is a commendable deed in our religion, because a mosque is a symbol of the House of God on Earth," he told the believers who gathered at the Kodeljevo Sports Hall in Ljubljana.
He also urged praying to God to protect the Muslims as a minority community in the world against violence and to bring peace to all the people in a time when the world is undergoing major changes.
The religious holiday Eid al-Adha is known among Muslims living in Slovenia, the majority of whom are of Bosnian descent, as Kurban Bayram.
The Muslim community organised morning prayers in almost 20 towns across the country.
