To express period and frequency—in other words, to say how many times something happened per day, week, and year, and so on—we need the a counter with the target particle. See here for an introduction to counters in Japanese.

【Period】【Number】【Counter】

This structure is most often used with the counter かい, which means “times” (e.g. I go back to my hometown 3 times per year”).

Note the position of the object particle—before the counter.

In fact, the target particle is often omitted.