"When will the Week of International Friendship be?"
Translation:Kiam estos la Semajno de Internacia Amikeco?
It's treating "Week of International Friendship" as a proper noun, a name, and capitalising the nouns in that phrase just as in the English.
Sort of like the difference between "in the united states" (common noun, any old states which happen to be united) and "in the United States" (proper noun/noun, a particular place which has the name 'United States').
So this is not just any week, it's a particular week which has the name "Week of International Friendship".
Because la Semajno de IA is the subject of okazos. (Who or what is taking place? The Week of IF is taking place.)
There's no reason for an accusative in this sentence; okazi is intransitive and so doesn't take a direct object in the accusative, nor is there a preposition here which would demand the accusative.