public class PlanesLogRow
{
    public DateTime ArriveDate;
    public string Origin;
    public string Destination;
}

var rez2 = planes.SelectMany(p => new[] {
         new { Airport = p.Origin, IsOrigin = true },
         new { Airport = p.Destination, IsOrigin = false }
       })
       .GroupBy(x => x.Airport)
       .Select(g => new
       {
           Airport = g.Key,
           LeftCount = g.Count(x => x.IsOrigin),
           ArrivedCount = g.Count(x => !x.IsOrigin)
       });