# FirstDayOfPeriod(Date date, Duration period, Date reference)

Returns the first day of the period in which the given date falls.

## Parameters

date
The date for which to calculate the first day of the period.
period
The duration representing the cycle of periods.
reference
The starting date of the sequence.

Date

## Remarks

The cycle of periods is defined by the given period duration, repeated in sequence starting from the provided reference date. If the period is not a whole number of days, the fractional part is ignored.

## Examples

To find the first day of this week, assuming the week starts on Sunday (using 1/1/2006 as the reference for Sunday), you can use:
`FirstDayOfPeriod(Today(), Days(7), Date(2006,1,1))`

For weeks starting on Monday, you can adjust the reference date, for example:
`FirstDayOfPeriod(Today(), Days(7), Date(2007,1,1))`

When setting up views that include specific date ranges, you can select "This Week", "Last Week", and "Next Week."

For columns named Date and checking for this/last/next week, you can use these formulas:

This Week:
`FirstDayOfPeriod([Date], Days(7), Date(2006,1,1)) = FirstDayOfPeriod(Today(), Days(7), Date(2006,1,1))`

Last Week:
`FirstDayOfPeriod([Date], Days(7), Date(2006,1,1)) = FirstDayOfPeriod(Today(), Days(7), Date(2006,1,1)) - Days(7)`

Next Week:
`FirstDayOfPeriod([Date], Days(7), Date(2006,1,1)) = FirstDayOfPeriod(Today(), Days(7), Date(2006,1,1)) + Days(7)`