年月日から曜日を調べるのも簡単です。
function weekday($y, $m, $d) { $date = New-Object System.DateTime($y, $m, $d) $date.DayOfWeek } function gen_1st_day_weekday() { foreach($y in 1901..2000) { foreach($m in 1..12) { weekday $y $m 1 } } } (gen_1st_day_weekday | where { $_ -eq 0 } | measure).Count