$date.Date.AddDays(-($date).DayOfWeek.value__)
PS C:\> $Date = Get-Date >> $date.Date.AddDays(-($date).DayOfWeek.value__) Sunday, March 24, 2019 12:00:00 AM
$date.Date.AddDays(-($date).DayOfWeek.value__)
ExamplePS C:\> $Date = Get-Date >> $date.Date.AddDays(-($date).DayOfWeek.value__) Sunday, March 24, 2019 12:00:00 AM