DateTime StartDate = "3/10/2009";
DateTime EndDate = "3/26/2009";
int DayInterval = 3;

int count = 0;

for(int i = 0; i < n; i++)
{
     count++;
     if(count >= DayInterval)
     {
          //take action
          count = 0;
     }

}

public IEnumerable<DateTime> EachDay(DateTime from, DateTime thru)
{
    for(var day = from.Date; day.Date <= thru.Date; day = day.AddDays(1))
        yield return day;
}

foreach (DateTime day in EachDay(StartDate, EndDate))
    // print it or whatever

foreach (DateTime date in StartDate.To(EndDate).ExcludeEnd()
                                   .Step(DayInterval.Days())
{
    // Do something with the date
}

DateTime StartDate = new DateTime(2009, 3, 10);
DateTime EndDate = new DateTime(2009, 3, 26);
int DayInterval = 3;

List<DateTime> dateList = new List<DateTime>();
while (StartDate.AddDays(DayInterval) <= EndDate)
{
   StartDate = StartDate.AddDays(DayInterval);
   dateList.Add(StartDate);
}

public static IEnumerable<DateTime> EachDay(DateTime from, DateTime thru)
{
    for (var day = from.Date; day.Date <= thru.Date; day = day.AddDays(1))
        yield return day;
}

public static IEnumerable<DateTime> EachMonth(DateTime from, DateTime thru)
{
    for (var month = from.Date; month.Date <= thru.Date || month.Month == thru.Month; month = month.AddMonths(1))
        yield return month;
}

public static IEnumerable<DateTime> EachDayTo(this DateTime dateFrom, DateTime dateTo)
{
    return EachDay(dateFrom, dateTo);
}

public static IEnumerable<DateTime> EachMonthTo(this DateTime dateFrom, DateTime dateTo)
{
    return EachMonth(dateFrom, dateTo);
}

DateTime startDate = new DateTime(2009, 3, 10);
DateTime stopDate = new DateTime(2009, 3, 26);
int interval = 3;

for (DateTime dateTime=startDate;
     dateTime < stopDate;
     dateTime += TimeSpan.FromDays(interval))
{

}

DateTime startDate = new DateTime(2009, 3, 10);
DateTime stopDate = new DateTime(2009, 3, 26);
int interval = 3;

while ((startDate = startDate.AddDays(interval)) <= stopDate)
{
    // do your thing
}

var today = DateTime.UtcNow;
var birthday = new DateTime(2018, 01, 01);

var toBirthday = today.RangeTo(birthday);

var toBirthday = today.RangeTo(birthday, x => x.AddMonths(2));

var toBirthday = today.RangeTo(birthday, x => x.AddYears(1));

// same result
var fromToday = birthday.RangeFrom(today);
var toBirthday = today.RangeTo(birthday);

public static DateTimeExtensions
{

    public static IEnumerable<DateTime> RangeTo(this DateTime from, DateTime to, Func<DateTime, DateTime> step = null)
    {
        if (step == null)
        {
            step = x => x.AddDays(1);
        }

        while (from < to)
        {
            yield return from;
            from = step(from);
        }
    }

    public static IEnumerable<DateTime> RangeFrom(this DateTime to, DateTime from, Func<DateTime, DateTime> step = null)
    {
        return from.RangeTo(to, step);
    }
}

// looping between date range
while (startDate <= endDate)
{
    //here will be your code block...

    startDate = startDate.AddDays(1);
}

while (startdate <= enddate)
{
    // do something with the startdate
    startdate = startdate.adddays(interval);
}