getStartOfWeek.ps1

function getStartOfWeek() {
    begin {
        $base = [DayOfWeek]::Monday
    }

    process {

        $difference = $_.DayOfWeek - $base
        $offset     = (7 + $difference) % 7 * -1
        
        $_.Date.AddDays($offset).Date
    }
}

[DateTime]::Now | getStartOfWeek