How do I iterate an IGrouping<T> Interface?

The IGrouping<TKey, TElement> interface inherits IEnumerable<TElement>:

foreach (var group in groupedData)
{
    var groupKey = group.Key;
    foreach (var groupedItem in group)
        DoSomethingWith(groupKey, groupedItem);
}

I note that you will be better off using this for your query, however:

var groupedData = Data.GroupBy(x => x.Period); 

rather than this:

var groupedData = Data.GroupBy(x => new {x.Period}); 

If, for example, you wanted to average the adjustments, you could do this:

foreach (var group in groupedData)
    Console.WriteLine("Period: {0}; average adjustment: {1}", group.Key, group.Average(i => i.Adjustment));

Each element of a sequence of IGrouping<TKey, TElement> is an IEnumerable<TElement> that you can iterate over to get the data that has a common TKey:

var groups = Data.GroupBy(x => x.Period);
foreach(var group in groups) {
    Console.WriteLine("Period: {0}", group.Key);
    foreach(var item in group) {
        Console.WriteLine("Adjustment: {0}", item.Adjustment);
    }
}

So in the above, groups is an IEnumerable<IGrouping<TPeriod, TAdjustment>> where TPeriod is the type of Period (you didn't tell us) and TAdjustment is the type of Adjustment. Then, group is an object that implements IEnumerable<TAdjustment> (but it also has a Key property so that you can get the key. Finally, item is a TAdjustment, and for each group, all the item that come from iterating over that group have the same key.

Though maybe obvious for others one can also use:

var groups = Data.GroupBy(x => x.Period);

foreach(var group in groups)
{
   List<Data> dataListByPeriod = group.ToList();
   //use this list
}