This is the 15413th most frequent Russian word.
праздничный
Festive.
The word "праздничный" here modifies the noun "ужин" (dinner), indicating that the dinner was in celebration or in a festive spirit.
Вчера мы устроили праздничный ужин.
Yesterday we had a festive dinner.
The word "праздничном" is the declined form of "праздничный" to match the prepositional case of the noun "мероприятии" (event), describing its celebratory nature.
На праздничном мероприятии было много гостей.
There were many guests at the festive event.
The word "праздничное" modifies the noun "платье" (dress), suggesting that the dress is suitable for a festive occasion.