This is the 5627th most frequent Russian word.
праздничную
Festive.
Here, 'праздничную' (festive) is used as an adjective describing 'одежду' (clothes), indicating a type suited for celebrations.
Я купил праздничную одежду для вечеринки.
I bought festive clothes for the party.
In this sentence, 'праздничную' (festive) modifies 'еду' (food), implying that the food is suitable for a special occasion.
Она приготовила праздничную еду для гостей.
She prepared festive food for the guests.
Here, 'праздничной' (festive) is an adjective agreeing with 'гирляндой' (garland), linking it to celebration themes.