Note: This release is in preview. Features described here don’t become generally available until the latest general availability date that Salesforce announces for this release. Before then, and where features are noted as beta, pilot, or developer preview, we can’t guarantee general availability within any particular time frame or at all. Make your purchase decisions only on the basis of generally available products and features.
Day in the Week, Month, Quarter, or Year
Example
Suppose that you want to see on which day of the week most deals are closed. Use day_in_week(date).
The resulting data displays the number of opportunities closed, grouped by the day of the week that the opportunities were closed on.

It looks like most opportunities are closed on Thursday (day 5).