Returns a date object of the first weekday of the month following another weekday, e.g. first Monday after Friday. In reply to https://www.reddit.com/r/PowerShell/comments/7bda01/calculate_first_monday_after_first_friday_of/

FirstWeekdayAfterWeekday.ps1

function Compare-WeekDays {
    # Return days between weekdays
    param (
        [Parameter(Mandatory=$true)]
        [System.DayOfWeek]$Minuend,
        [Parameter(Mandatory=$true)]
        [System.DayOfWeek]$Subtrahend
    )
    # In case they change how many days are in a week :)
    $DaysInWeek = [System.Enum]::GetNames('System.DayOfWeek').Count
    [int]$Difference = $Minuend - $Subtrahend
    if ($Difference -lt 0) { $Difference += $DaysInWeek }
    Write-Output $Difference
}

function Get-FirstWeekDayAfterWeekDay {
    param (
        [int]$Year = (Get-Date).Year,
        [int]$Month = (Get-Date).Month,
        [System.DayOfWeek]$FirstWeekDay = [System.DayOfWeek]::Monday,
        [System.DayOfWeek]$AfterWeekDay = [System.DayOfWeek]::Friday
    )
    $FirstWeekDayOfMonth = (Get-Date -Year $Year -Month $Month -Day 1).DayOfWeek
    $DayOfMonth = 1 +
        (Compare-WeekDays -Minuend $AfterWeekDay -Subtrahend $FirstWeekDayOfMonth) +
        (Compare-WeekDays -Minuend $FirstWeekDay -Subtrahend $AfterWeekDay)
    # if e.g. first Monday after Monday, need to add a week
    if ($FirstWeekDay -eq $AfterWeekDay) { $DayOfMonth += [System.Enum]::GetNames('System.DayOfWeek').Count }
    Write-Output (Get-Date -Year $Year -Month $Month -Day $DayOfMonth -Hour 0 -Minute 0 -Second 0 -Millisecond 0)
}