Simple logic to emulate running a script monthly.

runOnOccurance.ps1

# Set powershell job to run Weekly, on on the desired day.
# Wrap the code the job is running with the below so that it will only run 
# when intended.
# e.g. Last Wednesday of month:
#
# $DayToRun = 'Wednesday'
# Occurance = -1
#
# The scheduled job still runs once a week, but will only execute your code
# if the date is validated.

$Now = Get-Date
$DayToRun = 'Monday'
$OccuranceInMonth = 1 # It's an array so 1 = 2nd Occurance
$PossibleDays = @()

0..31 | ForEach-Object -Process {
    $TestDate = (Get-Date -Year $Now.Year -Month $Now.Month -Day 1).AddDays($_)
    if ($TestDate.Month -eq $Now.Month)
    {
        if ($TestDate.DayOfWeek -eq $DayToRun) {
            $PossibleDays += $TestDate.Day
        }
    }
}
$ValidRunDate = Get-Date -Year $Now.Year -Month $Now.Month -Day $PossibleDays[$OccuranceInMonth]

if ($Now.Date -eq $ValidRunDate.Date)
{
    # Put code here
}