> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt

# $isoDayOfWeek (expression operator)

## Definition

Returns the weekday number in ISO 8601 format, ranging from `1` (for Monday) to `7` (for Sunday).

The [`$isoDayOfWeek`](https://www.mongodb.com/docs/manual/reference/operator/aggregation/isodayofweek.md#mongodb-expression-exp.-isoDayOfWeek) expression has the following [operator expression syntax:](https://www.mongodb.com/docs/manual/reference/mql/expressions.md#std-label-aggregation-expressions)

```javascript
{ $isoDayOfWeek: <dateExpression> }
```

The argument can be:

- An [expression](https://www.mongodb.com/docs/manual/reference/mql/expressions.md#std-label-aggregation-expressions) that resolves to a [Date](https://www.mongodb.com/docs/manual/reference/bson-types.md#std-label-document-bson-type-date), a [Timestamp](https://www.mongodb.com/docs/manual/reference/bson-types.md#std-label-document-bson-type-timestamp), or an [ObjectID.](https://www.mongodb.com/docs/manual/reference/bson-types.md#std-label-document-bson-type-object-id)

- A document with this format:

  ```javascript
  { date: <dateExpression>, timezone: <tzExpression> }
  ```

  | Field | Description |
  | --- | --- |
  | `date` | The date to which the operator is applied. `<dateExpression>` must be a valid [expression](https://www.mongodb.com/docs/manual/reference/mql/expressions.md#std-label-aggregation-expressions) that resolves to a [Date](https://www.mongodb.com/docs/manual/reference/bson-types.md#std-label-document-bson-type-date), a [Timestamp](https://www.mongodb.com/docs/manual/reference/bson-types.md#std-label-document-bson-type-timestamp), or an [ObjectID.](https://www.mongodb.com/docs/manual/reference/bson-types.md#std-label-document-bson-type-object-id) |
  | `timezone` | Optional. The timezone of the operation result. `<tzExpression>` must be a valid [expression](https://www.mongodb.com/docs/manual/reference/mql/expressions.md#std-label-aggregation-expressions) that resolves to a string formatted as either an [Olson Timezone Identifier](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) or a [UTC Offset](https://en.wikipedia.org/wiki/List_of_UTC_time_offsets). If no `timezone` is provided, the result is in UTC. `Olson Timezone Identifier`:`"America/New_York"
  "Europe/London"
  "GMT"`; `UTC Offset`:`+/-[hh]:[mm], e.g. "+04:45"
  +/-[hh][mm], e.g. "-0530"
  +/-[hh], e.g. "+03"` |

## Behavior

| Example | Result |
| --- | --- |
| `{ $isoDayOfWeek: new Date("2016-01-01") }` | 5 |
| `{ $isoDayOfWeek: { date: new Date("Jan 7, 2003") } }` | 2 |
| `{ $isoDayOfWeek: {
    date: new Date("August 14, 2011"),
    timezone: "America/Chicago"
} }` | 7 |
| `{ $isoDayOfWeek: ISODate("1998-11-07T00:00:00Z") }` | 6 |
| `{ $isoDayOfWeek: {
    date: ISODate("1998-11-07T00:00:00Z"),
    timezone: "-0400"
} }` | 5 |
| `{ $isoDayOfWeek: "March 28, 1976" }` | `error` |
| `{ $isoDayOfWeek: Date("2016-01-01") }` | `error` |
| `{ $isoDayOfWeek: "2009-04-09" }` | `error` |

**Note: $isoDayOfWeek cannot take a string as an argument.**

## Example

A collection called `birthdays` contains the following documents:

```javascript
db.birthdays.insertMany( [
   { _id: 1, name: "Betty", birthday: ISODate("1993-09-21T00:00:00Z") },
   { _id: 2, name: "Veronica", birthday: ISODate("1981-11-07T00:00:00Z") }
] )
```

The following operation returns the weekday number for each `birthday` field.

```javascript
db.birthdays.aggregate( [
  {
    $project: {
      _id: 0,
      name: "$name",
      dayOfWeek: { $isoDayOfWeek: "$birthday" }
    }
  }
] )
```

The operation returns the following results:

```javascript
[
   { name: "Betty", dayOfWeek: 2 },
   { name: "Veronica", dayOfWeek: 6 }
]
```

**See also:**

- [$isoWeekYear (expression operator)](https://www.mongodb.com/docs/manual/reference/operator/aggregation/isoweekyear.md)

- [$isoWeek (expression operator)](https://www.mongodb.com/docs/manual/reference/operator/aggregation/isoweek.md)