LINQ to extract duplicate data more than 3 - list

Question

class Year
{
public int YearNumber;
public List<Month> Months = new List<Month>();
}
class Month
{
public int MonthNumber;
public List<Day> Days = new List<Day>();
}
class Day
{
public int DayNumber;
public string Event;
}
So I have a list of Years(list<year> years). How do I get the list (another list) which have the result that has duplicates event on the same day? I mean events can be happen on multiple dates, does not matter, what matters is, to find out if this any of date happens the same event from different year. . Lastly, (filter) only if its occurs more than 3 times. Example, 5 July 2014, 5 July 2017 and 5 July 2019 is 'Abc Festival', which occurs more than 3 times. So u get the date, the event, and the number of counts.

Answer 1

Using just the classes you show we can only group dates, where a "date" is a day in a month:
var query = from y in years
from m in y.Months
from d in m.Days
select new { m.MonthNumber, d.DayNumber }
into date
group date by date
into dateGroup
where dateGroup.Count() > 2
select dateGroup;
select dateGroup;
As you see, the core solution is to build new { m.MonthNumber, d.DayNumber } objects and group them.