I want to use `date_trunc` function in PostgreSQL on my datetime column to aggregate data in a week.

For example I need to get number of sales each week.

The problem is `date_trunc('week', datetime_column)` function considers Monday as the week start day and some of my customers user different start day in calendar (like Saturday).

I tried

```
 SELECT date_trunc('WEEK',(time_id + interval '2 day'))- interval '2 day' AS WEEK

```

but it's messy and I think there must be a better way.