Learn Croatian: Olimpijske igre održavaju se svake četvrte godine. - The Olympic games take place every fourth year.

While shorter repeating intervals like days or weekends often use the accusative (e.g., svaki drugi dan for 'every other day'), longer intervals like years or specific dates usually require the genitive case. So, for 'every fourth year', we use the feminine genitive singular for all three words: svake četvrte godine.

Why is the reflexive pronoun se used with održavaju here?

The verb održavati on its own means 'to hold' or 'to maintain' an event. Because the Olympic games are not holding something else, but rather 'are held' or 'take place', we use the reflexive form održavati se to convey this passive meaning.

Do ordinal numbers like četvrte decline like regular adjectives?

Yes, ordinal numbers in Croatian act just like definite adjectives. They must agree with the noun they modify in gender, number, and case. Here, četvrte takes the feminine genitive singular ending to match godine.

Could we say Olimpijska igra in the singular if we meant just one specific year's event?

No, the Olympic Games are always treated as a plural concept in Croatian (Olimpijske igre), exactly as they are in English. This is why the verb is in the third-person plural form, održavaju se.