Dates.fsx

let date year month day = new System.DateTime(year, month, day)

type DayOfWeek = | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday

exception DayOfWeekError

let dayOfWeek (date: System.DateTime) = 
    match date.DayOfWeek with
    | System.DayOfWeek.Monday -> Monday
    | System.DayOfWeek.Tuesday -> Tuesday
    | System.DayOfWeek.Wednesday -> Wednesday
    | System.DayOfWeek.Thursday -> Thursday
    | System.DayOfWeek.Friday -> Friday
    | System.DayOfWeek.Saturday -> Saturday
    | System.DayOfWeek.Sunday -> Sunday
    | _ -> raise DayOfWeekError

// Augusts with 5 3-day weekends

[2000 .. 3000]
|> List.filter (fun year -> (date year 8 1 |> dayOfWeek) = Friday)
|> List.iter (printf "%d; ")


let combine xs ys = [ for y in ys do for x in xs do yield y, x ]

let mmmyyyy (tw:System.IO.TextWriter) (date:System.DateTime) = tw.Write("{0:MMM yyyy}", date)

// Friday 13ths

[2014 .. 2020]
|> combine [1..12]
|> List.map (fun (year, month) -> date year month 13)
|> List.filter (dayOfWeek >> ((=) Friday))
|> List.iter (printf "%a; " mmmyyyy)