How to group by Year and Month?

I have DataTable with 2 columns: Id(long), Date(DataTime). How to group this to get grouped list in linq:

2017.08.03
      345675
      877685
2017.08.04
      988798
      898967
      675675

Next, I must to send each of group to stored procedure as a parameter. I can have about 300000 rows at data table so linq extension should be fast.

Ok. Now, I want to do something foreach groups. How Can I do it?

var GroupedbyDates = dataTable.AsEnumerable().GroupBy(x => ((DateTime)x["Date"]).Date);

 for (var i = 0; i < threadCount; i++)
 {
                Task task = Task.Factory.StartNew(DoSomething);
                tasks.Add(task);
 }

private DoSomething()
{
    while (true)
    {
        var item = ?? //one of groups;

         if (item != null)
         {
            var result = DoSomething2();
            HandleResult(result);
         }             
    }
}

How to iterate on IGrouped??

UPDATE

I have something like this:

var r = dataTable.AsEnumerable()
    .GroupBy(x => ((DateTime)x["Date"]).Year) 
    .ToDictionary(year => year.Key, g => g.GroupBy(x =>  
      ((DateTime)x["Date"]).Month)                                   
    .ToDictionary(month => month.Key,  d => d
    .Select(t => new { Root_Id = t["Id"] })));

to:

2017.08.03
      345675
      877685
2017.08.04
      988798
      898967
      675675