Version: 6.5.x and 6.7.x
The Day number function returns a number representing the weekday of the date passed in parameter. This value can take any value between 1 and 7, where 1 is Sunday and 7 is Saturday.
In the US calendar, Sunday is the first day of the week. However, if you export your 4D products to foreign countries such as European countries, you will need to know that they require the first day to be Monday. In this case, some adjustment to your code will be required.