Merge two arrays into one and get each item's occurrence in their parent array

Here I used a tuple (DateTime, int, int) to represent the data:

List<(DateTime, int, int)> result = firstDateArray.
            Concat(secondDateArray).
            Distinct().
            Select(x => (
                x,
                firstDateArray.Where(y => y == x).Count(),
                secondDateArray.Where(z => z == x).Count()
                )).
            ToList();

You can use .GroupBy, .Select, and .Join for this:

var today = new DateTime (year:2022, month:01, day:01);
var firstDateArray = new DateTime[] { today.AddDays(1), today.AddDays(7), today, today.AddDays(3), today };
var secondDateArray = new DateTime[] { today.AddDays(7), today, today.AddDays(3), today.AddDays(3), today.AddDays(1) };

var firstGrouped = firstDateArray
        .GroupBy(d => d)
        .Select(g => new { Date = g.Key, Count = g.Count() });
var secondGrouped  = secondDateArray
        .GroupBy(d => d)
    .Select(g => new { Date = g.Key, Count = g.Count() });

var joined = firstGrouped
    .Join(secondGrouped, f => f.Date, s => s.Date, (f, s) => new { Date = f.Date, FirstCount = f.Count, SecondCount = s.Count })
    .OrderBy(j => j.Date)
    .ToList();

GroupBy basically collects items with a common key (in this case the DateTime) and then lets you iterate through the collections.

Join takes 4 arguments: the second enumerable, the key selector for the first, the key selector for the second (it uses these to facilitate the join), and the result selector to produce the result you want.

Documentation:

• GroupBy
• Select
• Join
• OrderBy
• ToList

Try it online