# Excel WEEKNUM Function

The Excel WEEKNUM function takes a date and returns a week number (1-54) that corresponds to the week of year. The WEEKNUM function starts counting with the week that contains January 1. By default, weeks begin on Sunday.

**serial_num**- A valid Excel date in serial number format.**return_type**- [optional] The day the week begins. Default is 1.

The WEEKNUM function starts counting with the week that contains January 1. The **return_type** argument controls which day of the week is used to begin a new week number. Return_type is optional and defaults to 1.

With a return_type of 1-17, week number 1 in a given year is assigned to the week that contains January 1. With return_type 21, week 1 is the week containing the first Thursday of the year, following ISO 8601.

Return_type | Week begins |
---|---|

1 (default) | Sunday |

2 | Monday |

11 | Monday |

12 | Tuesday |

13 | Wednesday |

14 | Thursday |

15 | Friday |

16 | Saturday |

17 | Sunday |

21 | Monday (see note above) |

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