static void Main(string[] args)
{
    Execute(5);
    Execute(8);
    Execute(10);
    Execute(15);
}
static void Execute(int n)
{
    var dict =
        File.ReadLines($@"x{n}.csv")
            .Skip(1) // header
            .Select(l => l.Split(','))
            .ToDictionary(
                parts => new DateTime(year: int.Parse(parts[0].Trim('"')),
                                        month: int.Parse(parts[1].Trim('"')),
                                        day: 1),
                parts => int.Parse(parts[2].Trim('"')));

    int Get(DateTime dt) => dict.TryGetValue(dt, out int v) ? v : 0;

    var averagingKernel = new[] { 1, 3, 7, 3, 1 };
    var averagingKernelDict = Enumerable.Range(0, averagingKernel.Length)
        .ToDictionary(idx => idx - averagingKernel.Length, idx => averagingKernel[idx]);
    var kernelTotalWeight = averagingKernel.Sum();

    double GetAve(DateTime dt) => averagingKernelDict.Select(kvp => kvp.Value * Get(dt.AddMonths(kvp.Key))).Sum() / (double)kernelTotalWeight;

    using (var outf = File.CreateText($@"y{n}.csv"))
    {
        outf.WriteLine($"Date\tAveraged {n}+ Users Count");
        foreach (var key in dict.Keys.OrderBy(k => k))
            outf.WriteLine($"{key.Year}-{key.Month}\t{GetAve(key)}");
    }
}