How to group items by day, week and year

GroupByDateTime.cs

 IEnumerable<dynamic> GetGroup()
    {
        return settings.GroupBy switch
        {
            GroupByEnum.Day => listOfObjects.GroupBy(x => new DateTime(x.StartDate.Year, x.StartDate.Month, x.StartDate.Day))
                                                  .Select(x => new
                                                  {
                                                      Date = x.Key.ToString("d"),
                                                      Items = x
                                                  }),

            GroupByEnum.Week => listOfObjects.GroupBy(x => CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(x.StartDate, CalendarWeekRule.FirstDay, DayOfWeek.Monday))
                                                   .Select(x => new
                                                   {
                                                       Date = x.Count() == 1
                                                                ? $"{x.First().StartDate.ToString("d")}"
                                                                : $"{x.Last().StartDate.ToString("d")} - { x.First().StartDate.ToString("d") }",
                                                       Items = x
                                                   }),

            GroupByEnum.Month => listOfObjects.GroupBy(x => new DateTime(x.StartDate.Year, x.StartDate.Month, 1))
                                                  .Select(x => new
                                                  {
                                                      Date = x.Key.ToString("MMMM yyyy"),
                                                      Items = x
                                                  })
        };
    }