datestream.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ICantSeeSharp
{
    static class DateStream
    {
        public static IEnumerable<DateTime> AllDates
        {
            get {
                var theDate = DateTime.MinValue;
                while (theDate < DateTime.MaxValue.Date) {
                    yield return theDate;
                    theDate = theDate.AddDays(1);
                }
            }
        }
        public static Func<DateTime, bool> IsDayOfMonth(params int[] targets) => (DateTime actual) => targets.Contains(actual.Day);
        public static Func<DateTime, bool> IsDay(params DayOfWeek[] targets) => (DateTime actual) => targets.Contains(actual.DayOfWeek);
        public static Func<DateTime, bool> IsYear(params int[] target) => (DateTime actual) => target.Contains(actual.Year);
    }
}

namespace App
{
    using static ICantSeeSharp.DateStream;
    class Program
    {
        static void Main(string[] args)
        {
            var monOrWedStartOfMonth =
                AllDates
                    .Where(IsYear(2020))
                    .Where(IsDay(DayOfWeek.Monday, DayOfWeek.Wednesday))
                    .Where(IsDayOfMonth(1))
                    .ToArray();

            foreach (var date in monOrWedStartOfMonth)
            {
                System.Console.WriteLine($"{date} is {date.DayOfWeek}");
            }
        }
    }
}

datestream1.fs

open System

[<AutoOpen>]
module DateStream =
    let allDates = seq {
        let mutable theDate = DateTime.MinValue
        while theDate < DateTime.MaxValue.Date do
            theDate
            theDate <- theDate.AddDays 1.
    }

    let isDayOfMonth targets (actual:DateTime) = targets |> Seq.contains actual.Day
    let isDay targets (actual:DateTime) = targets |> Seq.contains actual.DayOfWeek
    let isYear targets (actual:DateTime) = targets |> Seq.contains actual.Year

open System

[<EntryPoint>]
let main argv =
    allDates
    |> Seq.filter (isYear [ 2020 ])
    |> Seq.filter (isDay [ DayOfWeek.Monday; DayOfWeek.Wednesday ])
    |> Seq.filter (isDayOfMonth [ 1 ])
    |> Seq.iter (printfn "%O")

    0