MesExtenso.cs

using System;
using System.Globalization;

static void Main(string[] args)
{
    var mes = ExibirMesPorExtenso(new DateTime(2019, 7, 7));

    Console.WriteLine(mes); //julho
    Console.ReadKey();
}

public static string ExibirMesPorExtenso(DateTime data)
{
    return data.ToString("MMMM", CultureInfo.CreateSpecificCulture("pt-br"));
}