Frequency of Events

There's a certain format to use when describing how often an event takes place. In English we say something like "I go to school three times a week." In Japanese, the format is roughly (period) に (frequency).

アリスちゃんは一週間に三回学校に行くと思う。
I think Alice-chan goes to school three times a week.

The 一週間 there is the period (one week), and the 三回 is the frequency (three times).

私は一ヶ月に二回シュウとごはんを食べるよ。
I eat lunch with Shuu twice a month.

私は一年に一回友達と日本に行く。
I go to Japan with my friends once a year.

ジムは一週間に五回髪を洗う、けど又臭いよね。
Jim washes his hair five times a week, but it's still smelly.