PowerShell Function: Get-WeekDayInMonth

Often, IT admins need to workout the first/second/third/fourth Mon/Tue/Wed/Thur/Fri/Sat/Sun of any given month. some good examples are:

• Prepare themselves for Microsoft’s patching Tuesday of each month
• Planning for any admin tasks caused by Day Light Saving time change

So I wrote this simple function today to calculate the date for any given month & year.

Here’s the function:

Function Get-WeekDayInMonth ([int]$Month, [int]$year, [int]$WeekNumber, [int]$WeekDay)
{
  $FirstDayOfMonth = Get-Date -Year $year -Month $Month -Day 1 -Hour 0 -Minute 0 -Second 0

  #First week day of the month (i.e. first monday of the month)
  [int]$FirstDayofMonthDay = $FirstDayOfMonth.DayOfWeek
  $Difference = $WeekDay - $FirstDayofMonthDay
  If ($Difference -lt 0)
  {
    $DaysToAdd = 7 - ($FirstDayofMonthDay - $WeekDay)
  } elseif ($difference -eq 0 )
  {
    $DaysToAdd = 0
  }else {
    $DaysToAdd = $Difference
  }
  $FirstWeekDayofMonth = $FirstDayOfMonth.AddDays($DaysToAdd)
  Remove-Variable DaysToAdd

  #Add Weeks
  $DaysToAdd = ($WeekNumber -1)*7
  $TheDay = $FirstWeekDayofMonth.AddDays($DaysToAdd)
  If (!($TheDay.Month -eq $Month -and $TheDay.Year -eq $Year))
  {
    $TheDay = $null
  }
  $TheDay
}

the $weekday variable represents the week day you after:

• 0: Sunday
• 1: Monday
• 2: Tuesday
• 3: Wednesday
• 4: Thursday
• 5: Friday
• 6: Saturday

Usage:

Example #1: to query the 2nd Tuesday of October 2012

Get-WeekDayInMonth –month 10 –year 2012 –Weeknumber 2 –Weeday 2

OR

Get-WeekDayInMonth 10 2012 2 2

Example #2: to query the 1st Sunday of May 2013

Get-WeekDayInMonth –month 5 –year 2013 –Weeknumber 1 –Weeday 0

OR

Get-WeekDayInMonth 5 2013 1 0