<#
  Public domain.

  Like any other piece of software (and information generally), this script
  comes with NO WARRANTY.

  This is version 20210426 of holidays.ps1.
#>
<#

  .SYNOPSIS
  Print a list of holidays for a given year and other good times to take a day off from work

  .DESCRIPTION
  Days off aren't just for weekends. Use this calendar of holidays to figure out the best days to take off from work.

  .PARAMETER Year
  The desired year to use when calculating holidays.

  .PARAMETER TimeZone
  The target time zone to use when calculating when Daylight Saving Time begins.
#>
Param(
  [Parameter(Mandatory=$False)] [UInt64] $Year     = (Get-Date).Year,
  [Parameter(Mandatory=$False)] [String] $TimeZone = 'Pacific Standard Time'
)
$ErrorActionPreference = 'Stop'
Set-StrictMode -Version:2

$holidays = @(
  @{ 'Name'        = "New Year's";
     'Month'       = 1;
     'Day'         = 1;
     'SpanFuture'  = 1;
  },
  @{ 'Name'        = 'Martin Luther King, Jr. Weekend';
     'Month'       = 1;
     'DayOfWeek'   = 1;
     'OrdinalWeek' = 3;
     'SpanFuture'  = 1;
     'SpanPast'    = 1;
   },
  @{ 'Name'        = "Valentine's Eve";
     'Month'       = 2;
     'Day'         = 13;
   },
  @{ 'Name'        = "Presidents' Day Weekend";
     'Month'       = 2;
     'DayOfWeek'   = 1;
     'OrdinalWeek' = 3;
     'SpanFuture'  = 0;
     'SpanPast'    = 1;
   },
  @{ 'Name'        = 'Leap Day';
     'Month'       = 2;
     'Day'         = 29;
   },
  @{ 'Name'        = "St. Patrick's Day";
     'Month'       = 3;
     'Day'         = '17';
   },
  @{ 'Name'        = "Monday after Mother's Day";
     'Month'       = 5;
     'DayOfWeek'   = 0;
     'OrdinalWeek' = 2;
     'SpanFuture'  = 1;
     'SpanPast'    = 0;
   },
  @{ 'Name'        = "Memorial Day Weekend";
     'Month'       = 5;
     'DayOfWeek'   = 1;
     'OrdinalWeek' = 'L';
     'SpanFuture'  = 1;
     'SpanPast'    = 1;
   },
  @{ 'Name'        = "Monday after Father's Day";
     'Month'       = 6;
     'DayOfWeek'   = 0;
     'OrdinalWeek' = 3;
     'SpanFuture'  = 1;
     'SpanPast'    = 0;
   },
  @{ 'Name'        = 'Independence Day';
     'Month'       = 7;
     'Day'         = 4;
     'SpanFuture'  = 1;
     'SpanPast'    = 1;
  },
  @{ 'Name'        = 'Labor Day Weekend';
     'Month'       = 9;
     'DayOfWeek'   = 1;
     'OrdinalWeek' = 1;
     'SpanFuture'  = 1;
     'SpanPast'    = 1;
  },
  @{ 'Name'        = 'Columbus Day Weekend';
     'Month'       = 10;
     'DayOfWeek'   = 1;
     'OrdinalWeek' = 2;
     'SpanFuture'  = 1;
     'SpanPast'    = 1;
  },
  @{ 'Name'        = 'Halloween Eve';
     'Month'       = 10;
     'Day'         = 30;
  },
  @{ 'Name'        = 'Veterans Day';
     'Month'       = 11;
     'Day'         = 11;
     'SpanFuture'  = 1;
     'SpanPast'    = 1;
  },
  @{ 'Name'        = 'Thanksgiving Weekend';
     'Month'       = 11;
     'DayOfWeek'   = 4;
     'OrdinalWeek' = 4;
     'SpanFuture'  = 2;
     'SpanPast'    = 1;
  }
  @{ 'Name'        = 'Christmas';
     'Month'       = 12;
     'Day'         = 25;
     'SpanFuture'  = 1;
     'SpanPast'    = 1;
  }
  @{ 'Name'        = "New Year's Eve";
     'Month'       = 12;
     'Day'         = 31;
  }
)

# Ported from libtai-0.60
Function Easter {
  Param(
    [Parameter(Mandatory=$False)] [UInt64] $Year
  )

  [Int32] $c = [Math]::Truncate($Year / 100) + 1
  [Int32] $j = $Year % 19
  [Int32] $t = 210 - ([Math]::Truncate(($c * 3) / 4) % 210)
  [Int32] $n = 57 - ((14 + $j * 11 + [Math]::Truncate(($c * 8 + 5) / 25) + $t) % 30)

  if (($n -eq 56) -and ($j -gt 10)) { $n-- }
  if ($n -eq 57) { $n-- }
  $n -= ([Math]::Truncate((($Year % 28) * 5) / 4) + $t + $n + 2) % 7

  if ($n -lt 32) {
    $month = 3
    $day   = $n
  } else {
    $month = 4
    $day   = $n - 31
  }

  $d = New-Object DateTime $Year, $month, $day
  Return $d
}

Function Main {

# Easter is always a Sunday
  $d = Easter -Year:$Year
  $holidays += @{
    'Name'       = 'Easter Weekend';
    'Month'      = $d.Month;
    'Day'        = $d.Day;
    'SpanFuture' = 1;
    'SpanPast'   = 1;
  }

# DST sucks
  $tz = [System.TimeZoneInfo]::FindSystemTimeZoneById($TimeZone)
  foreach ($rule in $tz.GetAdjustmentRules()) {
    if (($Year -ge $rule.DateStart.Year) -and ($Year -lt $rule.DateEnd.Year)) {
      if (-not $rule.DaylightTransitionStart.IsFixedDateRule) {
        $s = $rule.DaylightTransitionStart
        $holidays += @{
          'Name'        = 'Monday after Daylight Saving Time';
          'Month'       = $s.Month;
          'OrdinalWeek' = $s.Week;
          'DayOfWeek'   = $s.Day - 1;
          'SpanFuture'  = 1;
        }
      }
    }
  }

  $h = @()
  :LOOP foreach ($holiday in $holidays) {

    if (-not ([String]::IsNullOrEmpty($holiday['Day']))) {
      try {
        $t = New-Object DateTime $Year, $holiday['Month'], $holiday['Day']
      } catch { continue }
    }

    if (-not ([String]::IsNullOrEmpty($holiday['DayOfWeek']))) {
      $t = New-Object DateTime $Year, $holiday['Month'], 1
      if ('L' -eq $holiday['OrdinalWeek']) {
        $t = $t.AddMonths(1)
      }
      $n = [Byte] $t.DayOfWeek # 0 = Sun, 1 = Mon, 2 = Tue, etc.
      $o = ((7 + $holiday['DayOfWeek']) - $n) % 7
      if ('L' -eq $holiday['OrdinalWeek']) {
        $t = $t.AddDays(-1 * (7 - $o))
      } else {
        $t = $t.AddDays($o + (7 * ($holiday['OrdinalWeek']-1)))
      }
    }

    [Byte] $dow = $t.DayOfWeek

    if ((0 -ne $dow) -and (6 -ne $dow)) {
      $h += New-Object -TypeName:'PSObject' -Property:@{
        'Name' = $holiday['Name'];
        'DT'   = $t;
        'Date' = $t.ToString('D');
      }
    }

    if (-not ([String]::IsNullOrEmpty($holiday['SpanFuture']))) {
      $i = 0;
      $u = $t
      while ($i -lt $holiday['SpanFuture']) {
        $u = $u.AddDays(1)
        if ((0 -lt [Byte]$u.DayOfWeek) -and (6 -gt [Byte]$u.DayOfWeek)) {
          $h += New-Object -TypeName:'PSObject' -Property:@{
            'Name' = $holiday['Name'];
            'DT'   = $u;
            'Date' = $u.ToString("D");
          }
          $i++
        }
      }
    }

    if (-not ([String]::IsNullOrEmpty($holiday['SpanPast']))) {
      $i = 0;
      $u = $t
      while ($i -lt $holiday['SpanPast']) {
        $u = $u.AddDays(-1)
        if ((0 -lt [Byte]$u.DayOfWeek) -and (6 -gt [Byte]$u.DayOfWeek)) {
          $h += New-Object -TypeName:'PSObject' -Property:@{
            'Name' = $holiday['Name'];
            'DT'   = $u;
            'Date' = $u.ToString("D");
          }
          $i++
        }
      }
    }
  } # LOOP

  $out = @()
  $h | foreach {
    $out += New-Object -TypeName:PSObject -Property:@{
      'Name' = $_.Name;
      'DT'   = $_.DT;
      'Date' = $_.DT.ToString('D');
    }
  }
  Return ($out | Sort-Object -Property:'DT' | Select-Object -Property:Name,Date)
}
. Main
Exit