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】に【Number】【Counter】
This structure is most often used with the counter 回, which means “times” (e.g. I go back to my hometown 3 times per year”).
| 週に５回。 |
| Five times per week. |
| 年に２回に実家に帰る。 |
| To return to your hometown twice per year. |
| 周に本を３冊読みます。 |
| To read 3 books per week. |
Note the position of the object particle—before the counter.
In fact, the target particle is often omitted.
| 週４回ジムに通う。 |
| To go to the gym four times per week. |
| 年２回海外に行きます。 |
| To go abroad twice per year. |