Cannot Add Dates that are the 1st of a Month and on a Sunday to a DatetimeIndex
Running into a problem when I attempt to add the first date of a month that is a Sunday.

``` python
import datetime
from pandas.tseries.index import DatetimeIndex

d1 = datetime.date(2002, 9, 1)
d2 = datetime.date(2013, 10, 27)

d3 = datetime.date(2012, 9, 30)

i1 = DatetimeIndex([d1, d2])
i2 = DatetimeIndex([d3])

i1 + i2

ValueError: Could not evaluate WOM-1SUN
```

Returns a ValueError "Could not evaluate WOM-1SUN". However, if I change d1 and then follow the same steps 

``` python
d1 = datetime.date(2002, 9, 2)
i1 = DatetimeIndex([d1, d2])
i1 + i2

Out[2]: <class 'pandas.tseries.index.DatetimeIndex'>
[2002-09-02 00:00:00, ..., 2013-10-27 00:00:00]
Length: 3, Freq: None, Timezone: None
```

This create a DatetimeIndex adding the the index i1 and i2.
