Frequency tells us how often something is done.

(x times) a (time period)

One of the easiest ways to talk about frequency is the pattern:
(x times) a (time period).

x timestime period
one time / onceaday
two times / twiceaweek
three timesamonth
four timesayear

‘a’ = one
So in the sentence ‘once a day’, the thing is done one time in one day
= one time in each day

Examples using (x times) a (time period)

I have a birthday once a year.Michelle calls her mum twice a week.
Dave and I go to the pub three times a week. Louisa has work three afternoons a week.
The company has a general meeting four times a year.Byron changes his car once a decade.

(x times) every (y time periods)

If you want to talk about more than one (for example, 2 days, 4 weeks, or 5 months), change ‘a’ to ‘every’:
(x times) every (y time periods)

x timesy time periods
one time / onceevery2 days
two times / twiceevery4 weeks
three timesevery9 months
four timesevery10 years

In this pattern you can drop the words ‘one time’ or ‘once’.
For example:
I go to the park once every 2 weeks
= I go to the park every 2 weeks.

Examples using (x times) every (y time periods)

The Olympics are once every four years.When cooking the turkey, baste it once every 30 minutes.
The school has an activity day once every 3 months.James seems to have a new girlfriend every six months.
We talk on the phone three or four times every two weeks.The instructions say you should take this medicine three times every five days.

Comparing the patterns

I brush my teeth twice a day.
= two times in one day
I wash my clothes once every three days.
= one time in three days
Alan goes to school five times a week.
= five times in one week
Alan has an exam once every three months.
= one time in three months
We go to the cinema once a month.
= one time in one month
We meet twice every three weeks.
= two times in three weeks
Luke and Hassam go on holiday three times a year.
= three times in one year
Luke and Hassam get paid twice every six months.
= two tmes in six months
I get a haircut once a month.
= one time in one month
My girlfriend gets a haircut once every two weeks.
one time in two weeks