Distinct concat two IEnumerables and flag if it came from list B

Wood for the trees moment I'm sure.

var dateListA = new List<DateTime>();
var dateListB = new List<DateTime>();

for (var date = DateTime.Parse("01-Dec-2013"); date <= DateTime.Parse("08-Dec-2013"); date = date.AddDays(1))
    dateListA.Add(date);

for (var date = DateTime.Parse("05-Dec-2013"); date <= DateTime.Parse("10-Dec-2013"); date = date.AddDays(1))
    dateListB.Add(date);

var results = distinct_Merge_Of_dateListA_And_dateListB_But_With_New_WasInListB_Boolean_Property => new { Date, WasInListB };

foreach (var result in results)
{
    System.Console.WriteLine("{0} {1}", result.Date, result.WasInListB);
}

Expected output

01-Dec-2013 No
02-Dec-2013 No
03-Dec-2013 No
04-Dec-2013 No
05-Dec-2013 Yes
06-Dec-2013 Yes
07-Dec-2013 Yes
08-Dec-2013 Yes
09-Dec-2013 Yes
10-Dec-2013 Yes

I'm trying two merge to lists but indicate in the final list (with a boolean) if the result was included in list B. Any hints on how to do is are appreciated.