Wasei-eigo from English 'grand open'. Used for the opening of a new store, facility, or event, often with promotional fanfare.
wasei-eigo (Japanese-coined English), from grand open
The new shopping mall will have its grand opening next month.
We are holding a grand opening sale.
Variant with interpunct; less common but still seen in signage and promotional materials.
Wasei-eigo derived from English 'grand open', not standard English 'grand opening'.