Question

Is the word occasion a direct object, an indirect object, a predicate nominative, or a predicate adjective?

Answer 1

Heyo kiddo.

The answer is: Direct Object. 

Answer 2

Let's analize the phrase.

The Vietnamese people = subject (who?)

celebrate = verb

The occasion = Direct object (what? What do thry celebrate)

Remember, direct objects can always follow transitive verbs and they are never preceded by a preposition.